added properties that return the Nokogiri based nodes for selected elements

This commit adds two new properties that allow us to work directly with the selected `Nokogiri` based nodes that `wombat` parses. I have added the specs, as well as, the examples demonstrating the commit.

This is really useful for someone like me, who is more comfortable working with `Nokogiri` rather than `xpath`.

<em>Disclaimer: This is my first pull request, so please go easy on me :)</em>