Allow pulls from an insecure registry

Signed-off-by: Jason Bernardino Alonso <jalonso@luminoso.com>
Require docker-py 0.5 or later for insecure_registry kwarg

Signed-off-by: Jason Bernardino Alonso <jalonso@luminoso.com>
Add unit test for Service.pull

Signed-off-by: Moss Collum <mcollum@luminoso.com>
Merge pull request #490 from LuminosoInsight/insecure-pull

Allow pulls from an insecure registry