Issues/46 comments refresh

This branch does a few things:
- Refactors to have only a Comment model. AbstractComment and ReaderComment are removed from CoreData.
- Adds MigrationMapping to wrangle the changes to core data.
- Moves the comment syncing logic into the existing Comment service.
- Adds infinite scroll support to the reader comments. 
- Creates a sync helper class (in swift!) to wrangle syncing state.
- Adds support for estimated row heights and cached row heights to WPTableViewHandler.

Closes #46 
I feel the page calculation logic should be in the Service class. For a similar problem when syncing posts I did this https://github.com/wordpress-mobile/WordPress-iOS/commit/061319f709c9f94975c6fcac1fa96885cf5cedf1

Basically:
- Service has a `syncPosts` and `loadMorePosts`
- The service knows how to paginate depending on the Remote used