Add a prepend method to the verb class

Lets say your code runs through a lot of scenarios and returns a Response twiml object, and you want to add a particular verb to the start of the response Twiml.

    response = self.build_twiml()
    response.prepend(Play(url=audio_url))

Could use it for things like play a warning saying the call is going to be recorded before the dial. You might want to conditionally add that based on the response.