Import of 'JSQMessagesInputToolbar' instead of forward class declaration

Since 'JSQMessagesViewController' is meant to be an abstract class, it should import all of its dependencies in the header so that subclasses don't have to import them.

I came across this when my subclass of 'JSQMessagesViewController' did not recognize "self.inputToolbar.contentView". 
I think that subclasses shouldn't have to import specific header files for the abstract base class.