Added build task for creating jar of exoplayer library

Since it may be convenient to use a JAR of the Exoplayer library instead of as a library project (or Android Archive - aar), I've added a build task which creates a JAR of the library.

I've also updated the build system to use Gradle 0.12 instead of Gradle 0.10. This was done because the latest version of Android Studio (which is 0.8.2, at the time of writing) is not backward compatible with Gradle 0.10 anymore.