Updated docker logs timestamp to RFC3339

Currently the docker logs timestamp flag generates log entries like:

    $ sudo docker logs -ft daemon_dave
    [May 10 13:06:17.934] hello world

It uses Go's StampMilli timestamp to generate the timestamp. The entry
is also wrapped in [ ].

This is non-standard operational timestamp and one that will require
custom parsing.

The new timestamp is RFC3999 and generates entries like:

    2014-05-10T17:42:14Z hello world

These are readily parsed by tools like ELK.

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)