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 11d3edc that you fixed an issue with sorting. What was it?

To make sure that comments are sorted correctly I'm creating a string representation of their hierarchy and relying on a sort descriptor when performing a fetch.  I went with this approach since it allowed for inserting comments in the list without reordering the hierarchy. 

When sorting the strings "1", "2", "100" the sorted order would be "1", "100", "2" since in a string sort "100" comes before "2".  The padding was to prevent that. 

I looked at NSIndexPaths when I was considering which approach to use but I didn't see a way to make it work. Entirely possible that I just missed something.