cekees/enable shippable

After AGU I messed around a little bit with [docker](https://www.docker.com/) and found this relatively new continuous integration/deployment service  called  [shippable](https://www.shippable.com/) that integrates with docker and github. This PR does the following:

- builds  proteus using a hashdist cache built in the same docker container*
- runs `nose` with `coverage` enabled to generate coverage reports (https://app.shippable.com/projects/545531c244927f89db3e7d70)
- pushes the resulting docker container to [docker hub](https://registry.hub.docker.com/u/erdccm/proteus)

*right now I have to download  the hashdist cache because I'm using shippable's "minions".  There is a way to use your own docker container, which would just have the cache already, but you have to set up your own minion on rackspace or something.
In other words, I'd prefer that we keep the `proteus/test` directory structure.