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
Hmm, I think this should be broken up a bit: the uniter's the only thing that'll request a reboot (at least for now) but there'll be a distinct worker in the machine agent that watches for changes and gets the action, which should be on a facade specific to that worker; then I think it'll be the agent facade that's responsible for actually clearing the flag. Making sense?