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.
Just wondering, are we just being defensive here? As far as I understand, unsubscribing removes a listener already, so why use weak references?

Also, I don't think this should be static, as this means we're sharing mutable state with multiple instances of this class?