Add gradle build files; move root of app to /app subdirectory.

I added some Gradle build files and moved the app root to an 'app' subdirectory (makes working with Gradle builds more straightforward). I did not change any Java code or resource files. 'art' is still at the top level. This change should still be compatible with Eclipse (except now you have to import the project from /app subdirectory), but now the project is also accessible to Android Studio/Gradle users. The Gradle wrapper (gradlew/gradlew.bat) is auto-generated boilerplate and generally included for people who do not have Gradle installed.

I noticed /libs is in .gitignore; this means the dependencies of this project are not checked-in and cannot be known to people trying to work with this code. In particular, this project uses support Fragments so the support library is a required, but unspecified, dependency. I added this as a dependency in the Gradle build file, but I think you should just check in /libs to make this more obvious for people not using Gradle.