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 
If `NSIndexPath` is not the right fit, I'd add the current comment to the ancestors array and process them all:

```objc
NSArray *pathComponents = [ancestors arrayByAddingObject:[commentID stringValue]];
// Maybe rename method: it's not just ancestors if it includes the current ID
NSArray *arr = [self formatAncestorArray:pathComponents];
return [arr componentsJoinedByString:@"."];
```