Adds Vagrant configuration file

This commit makes it possible for a user to set up a fully working development environment for compiling the Bootstrap source files – and serving the documentation locally over port 9001. The only requirement is a working installation of Vagrant. All dependencies are installed automatically.

* start: `vagrant up`
* access bootstrap documentation on http://localhost:9001/
* stop: `vagrant destroy`

Learn more about Vagrant on http://www.vagrantup.com/.