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.
> Probably we should follow the conventions used in RxJava and name it something like OnSubscribeViewOnItemClick (and rename all other "operators" as well).
@mironov-nsk Agreed. But I think we should implement this in separate PR.