Updated JSQMessagesViewController to Version 6+

Same functionality as previously but with the updated JSQMessagesViewController.

![JSQMessagesViewController Version 6](https://s3.amazonaws.com/f.cl.ly/items/270B1o300H281X3T1e1m/iOS_Simulator_Screen_Shot_Dec_17__2014__5_13_41_PM.png)
@davidchiles This seems to be identical to the avatarImage method in OTRAccount. Can this be factored out into a shared utility method?