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? You're asserting that you're subscribing from the main UI thread already. AFAIK subscribeOn and observeOn ensure that the subscription is unsubscribed from on the same thread? So you either are already on that thread (as asserted above) or observeOn does that for you if you observeOn(mainThread())

Keep in mind that using the HandlerThreadScheduler means another roundtrip through the message loop, so this will unnecessarily delay unsubscription unless I'm missing something?