chore(fleetctl): consistent fleet logging

1. Use fmt.Printf for command output.
2. Use os.Exit to set return code.
3. Use glog for debug logging, with the verbosity level controlled with
a --debug flag. Everything should be logged using verbosity level 1
(i.e. glog.V(1).Infof) so it is off by default.
Merge pull request #273 from unihorn/56

chore(fleetctl): consistent fleet logging
style(fleetctl): add docstrings to readSSHChannel functions