Add setting to control the inclusion of Scala libs in the APK

After all that free advertising Facebook decided to give to our favorite Dalvik issue, I just learned that it had been fixed in recent versions of Android. Therefore, no more Proguard for development on Android 4.1+, which is amazing news!

Unfortunately the `android-plugin` absolutely wanted to skip the inclusion of the Scala libs in the generated APK when disabling Proguard. I added a setting key (`skip-scala-library`) to control that.

The default is "Use Proguard" and "Include Scala" (i.e. `useProguard := true` and `skipScalaLibrary := false`), which I believe is should work for most people.