Initial support for scheduling on Android Handler threads

To add to the discussion in https://github.com/Netflix/RxJava/pull/317, this is pretty much what we're using in our production app since a few weeks now. It used to live in our app project, but since we've already started talking about adding to rxjava-contrib, I've pulled out the code in question and integrated it into RxJava under rxjava-android.

This comprises:
- [x] a `Scheduler` implementation which schedules work on Android's `Handler` threads, including the main UI thread.
- [x] an `AndroidSchedulers` class which provides factory methods to instantiate Handler schedulers, especially the main thread scheduler
- [x] unit test support via Robolectric
- [x] build integration

There is some discussion needed around both test support and build integration. @mustafasezgin will want to weigh in on this. In order to write Android unit tests, you're unfortunately required to take a few obstacles, so we had to add some cruft to the production code (an empty Android `R` class and a custom test runner.)