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.
> I don't understand. Are you expecting that this operator is called multiple times for different listeners, i.e. will call be executed more than once on the same observable?

Yes, call will be executed more than once if user subscribes to the `Observable` multiple times.