A Dockerfile and fig.yml

Hello!

This is my first PR for OpenFarm. I've been following the project after contributed to kickstarter campaign.

I don't do much ruby or rails myself, but one of the more annoying steps to getting started on a project is getting the development environment setup. This PR adds a `Dockerfile` and a `fig.yml` which makes that step painless. You don't have to worry about installing the right versions of ruby, rails, or mongo. Just start up the docker containers!

If there are other external dependencies added (elasticsearch?) it will be easy to add them as well.

There are still a few TODOs, but I wanted to get this open to see how it's received.

TODOs:
* seed the mongo instance with some data
* merge or link to development docs from README.md