Drone-docker plugin

Basic drone-docker plugin work. Feedback appreciated.

Plugins in drone have the ability to modify the buildfile and add commands to be run
after the tests complete. This

This change adds a new Drone plugin for creating a docker image. The plugin
creates a docker image for the test build and pushes the image to images.docker.io.

I've just tested it end-to-end with 'drone build' from the command line because
I'm not sure of another test that would be useful.
@rgarcia I was having an issue building locally and I found a thread that said this pull request solved the issue (https://github.com/drone/drone/pull/264) so I copied the change over locally. I'm not sure why it hasn't been merged into drone master.

This shouldn't be necessary for my change to work, so I can remove it from the pull request. I wasn't sure what the best way to handle it was.