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.
@mttkay We're not defensive here, this is the only way I found to make multiple subscriptions work. There are two problems here:

- Methods like `setOnClickListener` and `setOnItemClickListener` allow to set only one listener, making multiple subscriptions hard to implement. This is the reason we need classes like `CompositeOnClickListener `.
- There is no way to get a `OnXXXListener` attached to the `View` (since there are no `getOnXXXListener` methods), so we need to save listeners somewhere else. `View.setTag(Object tag)` is not an option here since it's heavily used by the end developers. `View.setTag(int key, Object key)` could be a solution, but it requires a key to be declared in the resources, making it impossible to ship the library as a `jar`.

The only working solution I found is to store `CompositeXXXListeners` in the global `WeekHasMap` (it **have** to be `static`). Any other ideas how to solve this problem are welcome.