reboot worker

This is the final PR for the reboot functionallity. It add a Reboot worker that waits for the machines reboot flag to be changed, and reacts.

This branch depends on:

https://github.com/juju/juju/pull/707
https://github.com/juju/juju/pull/728
https://github.com/juju/juju/pull/744
gabriel/nate/horacio, what happened to the work that'd do all this setup at provisioning time, and subsequently always pull from agent config? the longer we leave it, the harder it'll be.