Add OperatorAdapterViewOnItemClick to observe OnItemClick events in AdapterViews. Resolve #19.

- new `OperatorAdapterViewOnItemClick` to handle onItemClick events from AdapterViews;
- separate `OnItemClickEvent` class to observe it;
- corresponding `itemClicks` method at `ViewObservable`;
- tests for ListView and GridView, also with multiple subscriptions.
Interesting, didn't know `unsubscribeOn`, thanks! What I was referring to is this bit:

https://github.com/ReactiveX/RxJava/blob/1.x/src/main/java/rx/internal/operators/OperatorObserveOn.java#L228