Add function to print out maze after each successful movement.

Okay, here it is! Nothing fancy, but it's a starting point.

Let me know what you think!

I also filled in ```lib/artoo-mazesolver.rb``` with the bare minimum necessary to get going, I think that would be a really useful starting point for "students" in the future. It essentially just moves randomly until the maze is solved...if you're interested I'd be willing to throw that into a separate pull request!