Add Ansible provisioner

This pull request adds a v2 provisioner that allows Vagrant to run [Ansible](http://ansible.cc) playbooks on the guest.  Though it supports a number of options, the simplest example of how to use it is:

    Vagrant.configure("2") do |config|
      config.vm.provision :ansible do |ansible|
        ansible.playbook = "playbook.yml"
      end
    end

With that, you can configure the guest using Ansible's powerful but simple playbook syntax. I find Ansible to be a great fit for Vagrant because it can do much more organized configuration management than the shell provisioner without all the moving parts of Chef or Puppet.  It's a middle ground that I think Vagrant does not yet have covered.

Also, I wrote up some documentation for this plugin in https://github.com/hashicorp/docs.vagrantup.com/pull/10.

Let me know if I can make any changes to make this provisioner better :) Looping in @mpdehaan in case he has any suggestions.