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.
From a naive standpoint, I would expect it to work like this:

```
ConnectableObservable source = sourceSequence.lift(new OperatorAdapterViewOnItemClick(listView)).publish();
source.connect();
source.subscribe(o1);
source.subscribe(o2);
```
but I might be missing something