GEB-299 Added attribute based methods to relative content navigators suc...

...h as next(), children() etc.

Just the *(Map<String, Object> attributes) methods for now. We'll talk more about the *(int index) methods!
We could name these methods `navigatorForRelativeElements()`, `navigatorForFollowingSiblings()`, etc - this would make the fact that they return a `Navigator` more obvious.