Provide sample Vagrant VM, provisioned via Ansible

To help document and automate deploying Thredded to a Ubuntu 12.04 VPS, the `contrib` directory now contains:

- A [`README`](https://github.com/shanesveller/thredded/blob/ansible-vagrant/contrib/README.md) documenting the commands to make use of this content
- A [`TODO`](https://github.com/shanesveller/thredded/blob/ansible-vagrant/contrib/TODO.md) documenting further plans for this PR
- A sample [Vagrantfile](http://vagrantup.com/) that is configured to use a Ubuntu 12.04 VM and provision it using [Ansible](https://github.com/shanesveller/thredded/blob/ansible-vagrant/contrib/www.ansibleworks.com/)

[Showterm screencast](http://showterm.io/8abb3e005072d9d23756b#fast)