More here: http://meedamian.com/post/publishing-with-gradle-revisited

.gitignore

# PROJECT `.gitignore`
# put it in the *root* of your project
# most likely just append this line to the already existing file 

/secret.properties

build.gradle

// MODULE `build.gradle`
apply plugin: 'com.android.application'

// read secret variables
// IMPORTANT: must be above `android {}` declaration
File secretPropsFile = file('../secret.properties')
if (secretPropsFile.exists()) {
    Properties p = new Properties()
    p.load(new FileInputStream(secretPropsFile))
    p.each { name, value ->
        safeLoad name as String, value
    }
}

android {
    compileSdkVersion 22
    buildToolsVersion "22.0.1"

    defaultConfig {
        applicationId "com.yourPackage.someMore"
        minSdkVersion 15 // because #minSDK15
        targetSdkVersion 22
        versionCode getBuildVersion(1)
        versionName "0.0.1"
    }
    
    // I usually use that, but you can safely remove that
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_7
        targetCompatibility JavaVersion.VERSION_1_7
    }

    signingConfigs {
        release {
            storeFile safeGetFile('STORE_FILE'))
            storePassword safeGet('STORE_PASSWORD')
            keyAlias safeGet('KEY_ALIAS')
            keyPassword safeGet('KEY_PASSWORD', safeGet('STORE_PASSWORD')) // notice how STORE_PASSWORD is used as a fallback
        }
    }

    buildTypes {
        release {
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
            signingConfig signingConfigs.release as Object
            minifyEnabled true
            zipAlignEnabled true

            File releasesDir = new File(
                safeGet('RELEASES_PARENT_DIR', '~/APKs'),
                safeGet('FOLDER_NAME', project.group as String)
            )
            if (!releasesDir.exists())
                releasesDir.mkdirs()

            android.applicationVariants.all { variant ->
                variant.outputs.each { output ->
                    if (output.name == "release") {

                        /* base file name in a form of:
                            [package]-[versionType]-[versionName]-[versionCode]
                            ex. com.meedamian.testApp-release-1.0.0-1111
                        */
                        String fileName = [
                            defaultConfig.applicationId,
                            project.name,
                            defaultConfig.versionName,
                            getBuildVersion(android.defaultConfig.versionCode, true)
                         ].join('-')

                        // rename output APK
                        output.outputFile = new File(releasesDir, fileName + '.apk')

                        // copy mappings.txt (JIC)
                        if (variant.getBuildType().isMinifyEnabled()) {

                            File mappingDir = new File(releasesDir, 'mappings')
                            if (!mappingDir.exists())
                                mappingDir.mkdirs()

                            assemble << {
                                copy {
                                    from variant.mappingFile
                                    into mappingDir
                                    rename 'mapping.txt', "mapping-${fileName}.txt"
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    
    // add your dependencies, ex:
    compile 'com.google.android.gms:play-services-base:7.0.0'
}

// get variables && prevent crashing if missing
String safeGet(String name, String defaultValue = '') {
    hasProperty(name) ? project[name] : defaultValue
}
File safeGetFile(String name) {
    String fileName = safeGet(name, null)
    fileName != null ? file(fileName) : null
}

// loads variables from a file to `project` so they can be `safeGet`-ed later
def safeLoad(String name, Object value, Boolean override = false) {
    if (!hasProperty(name) || override)
        project.set name, value
}

Integer getBuildVersion(defaultVersion, Boolean increment = false) {
    File verFile = file('../version.properties')

    if (!verFile.canRead())
        verFile.createNewFile()

    Properties props = new Properties()

    props.load new FileInputStream(verFile)
    String currentCodeVersion = props['VERSION_CODE']

    if (currentCodeVersion == null)
        currentCodeVersion = defaultVersion ?: android.defaultConfig.versionCode

    if (increment) {
        Integer bumpedCodeVersion = currentCodeVersion.toInteger() + 1
        android.defaultConfig.versionCode = bumpedCodeVersion

        props['VERSION_CODE'] = bumpedCodeVersion.toString()

        props.store verFile.newWriter(), "Build version updated with each release build"
        currentCodeVersion = bumpedCodeVersion
    }

    currentCodeVersion as Integer
}

GLOBAL_gradle.properties

# GLOBAL `gradle.properties`
# `~/.gradle/gradle.properties`
# 1. remove 'GLOBAL_' prefix from the name and
# 2. put it into ~/.gradle/ directory.

# for builds
RELEASES_PARENT_DIR=/Users/meeDamian/APKs

# for signing
STORE_FILE=/Users/meeDamian/keys/appSigningKey
STORE_PASSWORD=SuperSecretPasswordMickyGoofieConcretBananas

gradle.properties

# PROJECT `gradle.properties`
# put it in the *root* of your project.

# if not provided Project name will be used
FOLDER_NAME=projectName

# you have to provide that one
KEY_ALIAS=keyName

secret.properties

# PROJECT `secret.properties`
# if not provided store password will be used
KEY_PASSWORD=AnotherSuperSecretPassword