Add label and label-file

[nichivo]

I've been playing with traefik.io which makes use of container labels!

[nichivo]

I don't think changes in this patch cause the build to fail - it looks like go tip has changed a bit since fugu v1 was successfully built 11 months ago ;)

Should tip be removed from .travis.yml if fugu is just targeting 1.4? Or include something like this to ignore the build failure on tip but at least we'll know if it does work one day:

matrix:
  allow_failures:
    - go: tip

tip is a moving target anyway.

Or should we update fugu to work with 1.4 and tip?

Or can this pull request just be merged anyway? I only used go 1.4 ;)