Add docker.io Drone plugin which creates a docker image for each successful build

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.
Add tests for docker plugin

These tests just check that the docker commands show up in the buildfile
Merge pull request #1 from Clever/docker-plugin

Drone-docker plugin
Copying commit from https://github.com/drone/drone/pull/264
Change import so that it builds on my machine