Gradle update and optimisation (also fixes Android Studio building)

I updated Gradle to 1.12 and the Android gradle plugin to 0.10. Now Atarashii can be built again without troubles with the latest Android Studio 0.5.8 (as I mentioned in #99 Android Studio 0.5 introduced some gradle changes which caused some incompatibilities).
I've also done some optimisations to the gradle config:
- defined things required for all modules in the root build.gradle
- kept the wrapper tasks in the build.gradle files for easier update
  in the future