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
rebootuniter and reboot are in the same package. The API differs if you connect as a machine agent or if you connect as a unit agent. Both versions implement the "Rebooter" (I hate that name), which gets registered as a facade.