Configure the optional chef_solo provisioner in Vagrant
Also, support a better configuration syntax:

  Gusteau::Vagrant.detect(config) do |setup|
    setup.prefix = 'loco'
    setup.defaults.box_url = 'http://example.com/vm/opscode_centos-6.4.box'
    setup.provision = true
  end
Update README [ci skip]
Test underlying Vagrant configuration correctness
Wrap 'vagrant ssh' and 'vagrant provision'
Tweak README
Update README.md
Merge pull request #22 from locomote/vagrant_provision

Add an option to auto-configure Vagrant's chef_solo provisioner