WiP: Added reboot api client 

This is the third 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 PR depends on:

https://github.com/juju/juju/pull/707
https://github.com/juju/juju/pull/728
hmm, would you sync up with dimitern over this? I think he was doing something very similar, and was going to make the assigned machine available on the hook context anyway/