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
it would be good to have a docstring, even if it's trivial -- "RebootRequester implements the RequestReboot API method" (and similarly elsewhere)