Use custom Nib with JSQMessagesViewController subclass

This allows a custom nib to be used with a subclass of JSQMessagesViewController,

By implementing the `nibName` and `nibBundle` class methods in the subclass, a user can load a custom nib.

I really needed to use this myself, and I'm hoping it is allowed into the main repository.

If you are not sure why anyone would need to do this, I'm using it to load a subclass of the input toolbar that has different logic for when the send button should be enabled. In my application, I want users to only be able to reply to messages that were sent to them by us, but not initiate a conversation on their own, so I'm disabling the send button if there are no messages in the chat.