Add vagrant support

As an exercise in Vagrant, I added a Vagrantfile and provisioning script. They'll set up an environment necessary to locally serve a Jekyll page using the same settings as Github Pages. After typing 'vagrant up' and waiting patiently for ~30 minutes, you should be able to browse to http://127.0.0.1:4567/ and see the site. I think we can improve on this first attempt by using a box that already has some of the tools installed. Still, take a look and bring it in if you think it could be useful.
Let's use [rbenv](https://github.com/sstephenson/rbenv) with the [ruby-build](https://github.com/sstephenson/ruby-build) plugin instead of RVM. It's simpler and [less invasive](https://github.com/sstephenson/rbenv/wiki/Why-rbenv%3F).