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 
If `aPost == nil`, what probably happened is that the post got deleted. This should never happen as it's being displayed, but if it did, I don't think we'd want to pass the error. It looks like `WPContentSyncHelper` is discarding the error anyway, so no big deal