Implements webdriver timeout methods

These are options in selenium-webdriver, but haven't been encapsulated by watir-webdriver. I'm not sure why. If the intention is for this project to be a complete DSL, then I don't think we should ever need to reference driver in our tests.

I've liberally copied from selenium-webdriver in places (html files are exact copies). Do I need to do something special to maintain proper copyright/attribution?