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
if err := st.facade.FacadeCall("WatchForRebootEvent", nil, &result); err != nil {
    return nil, err
}