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
feat(fleetctl/journal): Add flag -f to follow journal
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
bump(github.com/coreos/go-systemd/dbus): 8abd78f0ac040cd0e8972866f912c03ba78945ee
fix(fleet): continue when failing to parse authorized_keys file

It should not panic the fleet.
chore(agent): minor adjustment on log output
chore(fleet): set verify_units to be false as default

This should have consistent default value with `verify` and `sign`
in fleetctl.
Merge pull request #189 from unihorn/24

chore(agent): minor adjustment on log output
Merge pull request #191 from unihorn/25

chore(fleet): set verify_units to be false as default
Merge pull request #187 from unihorn/22

fix(fleet): continue when failing to parse authorized_keys file
Merge pull request #186 from bcwaldon/bump-go-systemd

bump(github.com/coreos/go-systemd/dbus): 8abd78f0ac040cd0e8972866f912c03ba78945ee
fix(sign): handle the case that signature set is nil
refactor(verify): Use plural form of authorized keys

s/AuthorizedKeyFile/AuthorizedKeysFile/
s/authorized_key_file/authorized_keys_file/
Merge pull request #192 from bcwaldon/rename-authorized-keys

refactor(verify): Use plural form of authorized keys
doc(signing): Rewrite docs on payload signing
doc(signing): Update config docs for payload signing
Merge pull request #188 from unihorn/23

fix(sign): handle the case that signature set is nil