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 don't think we should be using `SafeExpectations` with the wp.com REST API, unless you explicitly want to convert types.

It made more sense in XML-RPC because deploying a "hotfix" could take 6+ months, but on WordPress.com we want it to fail as soon as possible and fix it. As stated a while back on an internal blog:

> ...when using the APIs please consult the documentation for what should be returned instead of introspecting the data for what is being returned. If you’re working in a language where that matters you should not hack it client side but instead fix the API to be correct