Add Fluent.switchTo() to change the WebDriver context into an iframe 

As seen in the Selenium FAQ : (https://code.google.com/p/selenium/wiki/FrequentlyAskedQuestions#Q:_How_do_I_type_into_a_contentEditable_iframe?), I implement a switchTo() method in Fluent.java to allow to change the Webdriver context to an iframe (restrict to an iframe for now).