Backed out changeset ec9fc64e82a8 (bug 940955) for landing with the wrong bug number
Bug 940954: Allow marionette to restart the browser and create a new session; r=jgriffin DONTBUILD

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()