Add compatibility with Jenkins Sauce ondemand plugin

Here are a few changes that I've been running to add compatibility with the Jenkins Sauce ondemand plugin:

* Use environment variables SAUCE_USER_NAME, SAUCE_API_KEY and SAUCE_ONDEMAND_BROWSERS
* Output Sauce session ID for reporting
* Refactor the code from pull request #17 to use the new browsers() function that was added to phpunit [recently](https://github.com/sebastianbergmann/phpunit-selenium/commit/ace30123ff64295a8e332950ec8eef19128a8a84)