Dockerized version

Related to #207.

You'll need to register at http://hub.docker.com/ and create automated build for this repo. The settings page should look like this:

![screen shot 2014-12-03 at 13 43 22](https://cloud.githubusercontent.com/assets/89186/5279125/6ac51bb6-7af2-11e4-8f7c-3cedf68b50b2.png)

When you plan to release new version, you have to change `ENV KOPF_VERSION` in `Dockerfile` before creating a tag. When the tag is ready, you need to add it in docker hub to create docker image for it.