Added support of the waitUntil() like Java and .NET client have

Currently it's not possible to test conveniently the cases when you need to wait until DOM element is removed from the DOM (implicitWait provides a solution for the opposite case)

So I added the waitUntil() support like in Java and C# .NET clients:

- http://seleniumhq.org/docs/04_webdriver_advanced.jsp#explicit-waits
- http://selenium.googlecode.com/svn/trunk/dotnet/src/WebDriver.Support/UI/WebDriverWait.cs
- http://selenium.googlecode.com/svn/trunk/java/client/src/org/openqa/selenium/support/ui/FluentWait.java