Bug 940955: Allow marionette to restart the browser and create a new session; r=jgriffin

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 : db352f88ebfc455ed8105eefac54fec6a0b0c40b