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 think we need this? 

The original `ViewObservable` operators don't have this check. I'm not sure why it was added after.

> AFAIK subscribeOn and observeOn ensure that the subscription is unsubscribed from on the same thread

There is an `Observable.unsubscribeOn(Scheduler)` operator.