Shuffle with Android-centric packages (#65)

This change introduces new, Android centric packages including

 * rx.android.content for android.content related classes
 * rx.android.view for android.view related classes
 * rx.android.widget for android.widget related classes
 * rx.android.util for package agnostic classes

Then it moves existing classes based on their Android counterparts.

Although one of the goal is to reduce the number of publics,
this change still does nothing but class move. More changes will follow.
Split AndroidObservable to ContentObservable and ViewObservable (#65)

This allows some contnet-related internal classes package local.
Split part of ViewObservable to WidgetObservable. (#65)

Some ViewObservable methods are for android.widget.* classes.
This change moves  these to newly introduced WidgetObservable.
This turns some internal classes from public to package-local.
Move Assertions from util to internal.