Make it easier to get started as a contributor.

I was having some trouble getting the dependencies (MongoDB, Ruby 2.0.0) installed on my local machine; this PR adds a `Vagrantfile` describing a VM with all of the dependencies set up and installed correctly. It is not necessary to use Vagrant to contribute but it may make it easier for some developers. This PR also updates the `README.md` with clearer instructions on how to get started contributing to the project.