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
fix(fleetctl): tweak wording of blocking start output

A unit hasn't necessarily started when the blocking command
exits, it has only been scheduled. It can still fail to start
on the target machine.
Merge pull request #278 from bcwaldon/fleetctl-wording

fix(fleetctl): tweak wording of blocking start output
chore: gofmt all go files
Merge pull request #280 from unihorn/58

chore: gofmt all go files
feat(test): test code format