Added Depth First Search in Lua

Hi,

Submitting a new graph traversal algorithm, [Depth First Search](http://en.wikipedia.org/wiki/Depth-first_search) in Lua. It is a generic implementation, and it includes some handlers for reference.
Tests have also been included.

PS: I also included some fixes for the previous implementations. I might send further pushed as enhancements, as I am starting to see some flaws in the way I devised my handlers. But for now on, they work fine.

Regards,
Roland.