Added Vagrant configuration file

This pull 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__ other dependencies are installed automatically making Bootstrap development super easy!

* start: `vagrant up`
* access bootstrap documentation on http://localhost:9001/
* stop: `vagrant destroy`

Learn more about Vagrant on http://www.vagrantup.com/.