Add system time to /info

This change adds daemon's system time as RFC3339Nano to the `/info` endpoint
and shows in a more readable format (UnixDate) in `docker -D info` output.

I will be using this to fix the clock skew between the remote test host and
the CI machines running `docker events`-related tests as they're using `--since`
and `--until` and the timestamps are not matching when daemon is not on the
same machine.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

docs/sources/reference/commandline/cli.md

@@ -1248,6 +1248,7 @@ For example:
     Debug mode (client): true
     Fds: 10
     Goroutines: 9
+    System Time: Tue Mar 10 18:38:57 UTC 2015
     EventsListeners: 0
     Init Path: /usr/bin/docker
     Docker Root Dir: /var/lib/docker