Now dynamically finds a HtmlUnitDriver on the CP

Instead of instantiating a HtmlUnitDriver class directly, the code now
attempts to initialize the default driver with a HtmlUnitDriver via
name, hoping to pick whatever version is available at the classpath.
Added :contains() selector
Selenium version from 2.40.0 to 2.32.0 for bwd compatibility

Changed selenium version from 2.40.0 to 2.32.0 for easier backward
compatibility