Add top content inset support

I added a simple property that is supposed to be set before [super viewDidLoad] is called on the class that inherits from JSQMessagesViewController. When tried this for the first time, the scrolling to bottom functionality didn't work, so I fixed it by using a scrollRectToVisible:animated: method call. My particular need was to have a view on top of the collection view, and have the top contentInset of the collection view value to take this into account.
@gfzabarino hm... was the problem #256 ? does this fix #256 ?