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
you mean for RequestReboot via uniter?

https://github.com/juju/juju/pull/744/files#diff-fb67fb6a49fa075cdc2b7b31d9d25374R44