Add Vagrant configuration.

capybara-webkit has various development dependencies which can be a little
cumbersome to manage. With [Vagrant](http://www.vagrantup.com) they can all be nicely bundled up in a VM and
installed automatically. My personal preference is to use Vagrant for anything that depends on anything outside of its Gemfile (in this case Qt and ImageMagick).

It's hard to test this automatically, but it can be manually tested using something along these lines:

    gem install vagrant
    cd capybara-webkit

    # First run will set up the VM and run vagrant_setup.sh
    vagrant up

    # Run the capybara-webkit tests on the VM
    vagrant ssh
    cd /vagrant
    rake
    exit

    # Restart the VM, second run of vagrant_setup.sh will be much quicker
    vagrant halt
    vagrant up

    # Run the capybara-webkit tests on the VM
    vagrant ssh
    cd /vagrant
    rake
    exit