Add external sources to javadoc.

javadoc.gradle

apply plugin: 'java'

repositories {
  mavenCentral() 
}

dependencies {
  compile group: 'net.spy', name:'spymemcached', version: '2.10.3'
}

javadoc {
  def sources  = configurations.create '__depSources'

  dependencies {
    __depSources group: 'net.spy', name:'spymemcached', version: '2.10.3', classifier: 'sources'
  }

  def jars = configurations.depSources.files
  jars
    .collect({ jar -> project.zipTree(jar).matching({ include '**/*.java'}) })
    .each { files -> 
      source += files
    } 
}