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 
In other words, I still think we should be "liberal on what you accept from others" but strict in what we accept from ourselves. 

We're all scarred from a long time of dealing with XML-RPC, but I believe we should trust the documentation and bet on API correctness. If things start to blow up the blame is on the API side.

We're probably the biggest consumer of the wp.com API and I feel like if we can't trust it to work properly we can't expect others to do it. It feels risky because it is, but hiding the problems is also risky :)