Adding jquery helpers for Selenium testing

I wrote this little tidbit today at work while working with Selenium.  It's not a substantial enhancement, but I think it kicks the Selenium DSL story up a notch.  With this enhancement you can write tests like this:

```scala
class BlogSpec extends FlatSpec with ShouldMatchers with WebBrowser with Eventually {
  implicit val webDriver: WebDriver = new HtmlUnitDriver
  import jquery._

  go to "https://twitter.com"
  $("#signin-email").click()
  pressKeys("joescii")
  $(".primary-btn").click()

  eventually {
    $$(".tweet-text").foreach(tweet => // ...
    )
    // OR
    $("#timeline").findAll(".tweet-text").foreach(tweet => // ...
    )
  }
}
```

Anyway, it's here if you decide you'd like to include it in scalatest.  I'll gladly contribute to updating the **Using Selenium** section of the guide.  Thanks for all of the great work!