Unity script to increment the version number each time the project is run in the editor, the version can be queried at run time with 'Application.version'.

VersionIncrement.cs

// Minor adjustments by Arshd
// Version Incrementor Script for Unity by Francesco Forno (Fornetto Games)
// Inspired by http://forum.unity3d.com/threads/automatic-version-increment-script.144917/
// Modified by HJ

using UnityEditor;
using UnityEditor.Callbacks;
using UnityEngine;
using System;

[InitializeOnLoad]
public class VersionIncrement
{

    static VersionIncrement()
    {
        EditorApplication.update += RunOnce;
    }

    static void RunOnce()
    {
        EditorApplication.update -= RunOnce;
        IncreaseBuild();
    }

    static void IncrementVersion(int majorIncr, int minorIncr, int buildIncr)
    {
        string[] lines = PlayerSettings.bundleVersion.Split('.');

        int MajorVersion = int.Parse(lines[0]) + majorIncr;
        int MinorVersion = int.Parse(lines[1]) + minorIncr;
        int Build = 0;
        if (lines.Length > 2)
        {
            Build = int.Parse(lines[2]) + buildIncr;
        }
        string date = DateTime.Now.ToString("yyyyMMddhhmmss");

        PlayerSettings.bundleVersion = MajorVersion.ToString("0") + "." +
                                        MinorVersion.ToString("0") + "." +
                                        Build.ToString("0") + "." + date;
        //PlayerSettings.Android.bundleVersionCode = MajorVersion * 10000 + MinorVersion * 1000 + Build;
    }

    [MenuItem("Build/Increase Minor Version")]
    private static void IncreaseMinor()
    {
        IncrementVersion(0, 1, 0);
    }

    [MenuItem("Build/Increase Major Version")]
    private static void IncreaseMajor()
    {
        IncrementVersion(1, 0, 0);
    }

    private static void IncreaseBuild()
    {
        IncrementVersion(0, 0, 1);
    }
}