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.
makes sense. What do you think about moving this commit to a different branch so that we have the following branches:

* master: in sync with drone/drone#master
* docker-plugin: *just* the addition of the docker plugin so that we can make a clean PR on upstream drone
* clever: any other changes we've made to build drone for our own purposes (we'll use this branch to deploy to our own infrastructure)