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 
Actually, I'm not sure what the padding is for. I see in 11d3edcc5bc335c6a566879ec89a22304d6a1655 that you fixed an issue with sorting. What was it?

What if `hierarchy` was stored as a `NSIndexPath`, would that help?