Android-centric package reorganization (#65)

Here is a first cut.

What seems good for me:

* Less public classes
* Less imports 

What doesn't look great but not sure how can be better:

* Breaking change!
* New facade classes (WigetObservable, ContentObservable) - Feels a bit awkward. Probably better to split into per-counter part classes like TextViewObservable?
* rx.observable.AndroidSubscriptions - Feels there could be better home for this guy, but I'm not sure where it is :-/

WDYT? Comments are appreciated.

Should this be in an internal package? I don't think it's our responsibility to expose this in a public API.