Trying to improve the module setups.

I'm still not sure what the best way is to do this is yet.

This introduces the ability to build a "rebound.jar". It also renames
the android library to be "rebound_android".

To get this working in an Android project, you'd have to build both
":rebound" and "//src/com/facebook/rebound/android:rebound_android", and
include both of the JARs (buck-out/gen/rebound.jar and
buck-out/gen/src/com/facebook/rebound/android/lib__rebound_android__output/rebound_android.jar)
inr your project.

Doesn't seem super intuitive?

Also didn't see a straightforward to have rebound_android.jar bundle
rebound.jar inside itself, using Buck.