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
yeah, please never assume anything about what's "current" -- always take an array of independent requests, even if it's not immediately apparent why you'd want to.

(in the set-reboot case in particular, I think we'll *definitely* want to be able to set the reboot flag for N machines at once, at some point -- probably via the client API -- so it's probably worth going ahead and implementing that as an embeddable RebootRequester that we can start off embedding in the Uniter facade, and add to a new client facade when we turn out to need it. By keeping the auth controlled by the embedding facade, we should be able to do this really easily)