Allow to use a configured WebDriver when running with test browser.

Helpers.running so far accepted a WebDriver class to instantiate
the appropriate WebDriver.

This change allows to provide an already configured WebDriver instance.
This makes it possible to e.g. create an HtmlUnitDriver for a specific
browser version, like this:

    import org.openqa.selenium.htmlunit._
    import com.gargoylesoftware.htmlunit._
    val driver = new HtmlUnitDriver(BrowserVersion.FIREFOX_17)
    driver.setJavascriptEnabled(true)

    running(TestServer(3333), driver) { browser =>
      ..
    }

Actually for me using the BrowserVersion.FIREFOX_17 fixed an issue
with jquery: integration tests failed with

    WebDriverException: com.gargoylesoftware.htmlunit.ScriptException: TypeError: Cannot find
    function addEventListener in object [object HTMLDocument]. (http://code.jquery.com/jquery-1.10.1.min.js#5)