Added Vagrantfile which sets up the essential environment for `phpunit-selenium` hacking

I've added a `Vagrantfile` that sets up the complete environment to develop and test `phpunit-selenium`

It includes:

* headless `firefox` (via `xvfb`)
* `selenium-server-standalone-2.41.0` (there are some issues with `v2.42`, see https://github.com/sebastianbergmann/phpunit-selenium/pull/311)
* `php5` and `composer`
* `supervisord` to run `selenium` and `xvfb`

Known issues:

* Currently one test for `Selenium2TestCaseTest.php` fails, I will check why it does soon
* Some other fails for `Selenium2TestCase` directory (but to be honest I have never seen them working)