Added support for Android Studio

Hello,

I added Android Studio/gradle support. Mainly just a copy paste of the build.gradle from http://tools.android.com/tech-docs/new-build-system/migrating-from-intellij-projects.

Needed to add tools:overrideLibrary="android.support.v13" to the AndroidManifest.xml. as the v13 support library minimum APK is higher than pgmp's minimum apk, which caused manifest merging failures.