WiP: Added apiserver endpoint for reboot

This is the second pull request for the reboot implementation. Please note that this is still a Work in Progress and a followup commit will be done for cleanup.

This branch depends on: https://github.com/juju/juju/pull/707
blast, sorry I didn't see this -- but shouldn't this be on the agent facade, not the machiner? the machiner is pretty limited and specific.

This should just be a matter of moving the code/tests, though.