feat(signing): add signing and verification for unit files in etcd

The patch is done for security reason.

Details are in Documentation/sign.md
chore(fleetctl): use fmt.Print() instead of print()

As http://golang.org/ref/spec#Bootstrapping says, print is documented for
completeness but is not guaranteed to stay in the language.
Replace it with fmt.Print
Merge pull request #182 from unihorn/22

feat(fleetctl/journal): Add flag -f to follow journal
Merge pull request #181 from unihorn/21

chore(fleetctl): use fmt.Print() instead of print()
Merge pull request #172 from unihorn/18

feat(signing): add signing and verification for unit files in etcd
fix(fleet/verify_units): continue when failing to parse authorized_keys file