Extend JSQMessageData to represent an image

Summary:
1. Extend the JSQMessageData interface to include a type & UIImage accessor
2. JSQMessagesViewController sets the image if the type permits
3. JSQMessagesCollectionViewFlowLayout calculates the size based upon the image if the type permits
4. Added a UIImageView property to JSQMessagesCollectionViewCell & 2 concrete XIBs

Notice that the XIB had their "toolsVersion" changed.
Perhaps this is because I am using an xcode that is newer/older than the one you are.
I'm using Version 6.0.1 (6A317)