adds support for PhantomJS to PropertyWebDriverProvider

JBehave/Selenium has been integrated with PhantomJS via [GhostDriver](https://github.com/detro/ghostdriver).

This pull request adds support for specifying -Dbrowser=phantomjs to the PropertyWebDriverProvider class.

I've added a test too but it's disabled as you need to have the phantomjs binary on your $PATH for it to run...