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 
This is probably me just not being able to read Swift yet, but wouldn't this call a delegate method named `hasNoMoreContent` every time the variable is set, regardless of its value?