This adds a simple Vagrantfile to the root directory, which you can use
to easily spin up RHEL/Debian/Ubuntu virtual machine in seconds to test
GHC.

For example, from the root of the GHC tree, you can say:

    $ vagrant up ubuntu1204-amd64
    $ vagrant ssh ubuntu1204-amd64

This will give you access to a provisioned Ubuntu 12.04 virtual machine
with all the necessary GHC dependencies installed (modulo a few things).

Debian 7/CentOS 6.5/Ubuntu 12.04 only for now, in amd64/i386
configurations. In the future I plan to at least add FreeBSD and NixOS
boxes where possible.

Improvements are necessary of course. By default a box is given 4GB of
RAM and 2 cores, and resolves DNS entries by routing through the host
DNS configuration. Do not run 'vagrant up' unless you have lots of RAM
as it will spawn *every* virtual machine.

Hopefully, this should make it far easier for contributors to get
started eventually.

Signed-off-by: Austin Seipp <austin@well-typed.com>