Converted build system to gradle

This is the new Android build system. The reason I converted is that I started using Android Studio and its preferred way of building is gradle.

@matburt Would you be willing to change the build system? You would need to edit the MobileOrg/build.gradle with accordance to http://tools.android.com/tech-docs/new-build-system/user-guide#TOC-Signing-Configurations I created a signing config called matburt that you just need to fill out. Creating the donate and release versions is done by running ./gradlew assembleRelease or ./gradlew assembleDonate.