[rxjava-android] OperationObserveFromAndroidComponent

This is in reference to https://github.com/soundcloud/RxJava/issues/1

Android applications can use this operator to safely observe a source sequence on the main user interface thread. As long as the subscription is unsubscribed from in `onDestroy` or `onDetach`, no resource leaks will occur.

I have also added an `AndroidObservable` class which acts as an entry point for instantiating Android specific observer instances.