add isFocused() on Navigator

Originally created by Marcin Erdmann.

It would be cool not to have to use javascript or WebDriver's api to check if an element has focus.

We should probably be using http://selenium.googlecode.com/svn/trunk/docs/api/java/org/openqa/selenium/WebDriver.TargetLocator.html#activeElement(), just that the API is not straightforward so it won't be trivial to achieve that
