[WIP] REST comments sync

**DO NOT MERGE**

This hasn't been tested yet, and is not finished, but I created the pull request to get some feedback on how the code is being refactored.
Actually, the method already is `dateWithWordPressComJSONString`, since we might need a slightly different format if we implement the core JSON API, which would go in the same `NSDate (JSON)` category, hence the `JSON` name. So `WordPressJSON` might work better :smile: