Bug 940954: Allow marionette to restart the browser and create a new session

This gives us the ability to restart a session from the client side, say for testing Firefox updates,
and then carry on the test. To do this it is just self.marionette.restart()

--HG--
extra : rebase_source : 95d0c6d4fcd30865ff7cbbf97de0e5ca09b3eaab