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.
can we break up this test into individual isolated scenarios? It's difficult to understand from looking at the test what it achieves