Extract a more full featured Post Object

[regnerjr]

There is the beginnings of one here: d8bccca#diff-e463df27f983f6705de18b276c51cda9R86

But it does not yet have all the required properties, and is not used throughout.
It need to be integrated better in the post view controller: https://github.com/iOS-Connect/parse-twitter-with-push/blob/master/Parse-Twitter/Parse-Twitter/PostViewController.swift#L17