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.
Regarding the question why it have to be a `WeakHashMap`. Well, it could be just a usual `HashMap`, but in this case user **have** to unsubscribe from the `Observable  in order to prevent memory leaks`, making the usage of `ViewObservables`s a bit more complicated.