Add rxjava-android build module + config

Should this be shared with other contribs through the parent module? Lots of copy and paste here.
Add HandlerThreadScheduler which schedules actions on an Android Handler thread
Expose main thread scheduler through AndroidSchedulers class
Add support for running Robolectric tests
Add unit tests for HandlerThreadScheduler
Reduce duplication by making "schedule now" the special case

Forwards to "schedule later" with delay of 0 now.
Add empty .gitignore file so that git checks out the values folder
Moved sources under main android package.
Excluded the test support package from being included when building a jar or generating javadoc
Javadoc comments added
Removed the exclude from the javadoc task as it was breaking javadoc generation from the projects root folder
Upgrade to Robolectric 2.0 and remove all the things...
Merge pull request #318 from mttkay/android

Initial support for scheduling on Android Handler threads