05-paging

[Serchinastico]

This PR is not meant to be merged

This is how to use the Paging component:

• Return PagedList instead of a simple List of elements (in this case, SuperHero). Room already gives support to it and deals with pagination for you but it's up to the developer to have a backing data source that supports pagination.
• Create a BoundaryCallback implementation. It is called whenever the LivePagedListBuilder created in the local data source runs out of elements. It has to fetch the new elements from somewhere, in our case from the RemoteSuperHeroDataSource and store them in the LocalSuperHeroDataSource. The API is too simple as it only returns the last item it found, that means that we have to deal with knowing what page is that in our system. In this PR we are just storing an index that is increasing with every new fetch but if the application would be more complex we'd have to create a fancier mechanism. BTW, the callback is a repository dependency because if I'd add it as a dependency of the local data source I'd be creating a dependency cycle.
• Replace the regular RecyclerView.Adapter by a PagedListAdapter. It is this element the one really firing the event in case we need more elements. It relies on you calling the getItem method so you are forced to call it from your implementation adapter even if it's not used.
• Because the paging adapter is using the diff'ing algorithm we are adding a short sleep in our tests to wait for animations to finish.

[davideme]

Great job @Serchinastico !!!!