Add a quadtree

[luizbills]

No description provided.

[Add00]

I've been interested in creating a quadtree implementation for sometime. Could I add this feature?

[Add00]

I was too interested and when ahead and created. I hope that isn't problematic.

[luizbills]

There are just a few issues: you made the changes on the same branch, so both of your pull requests #19 and #18 have the same stuff. Use two different branches.

About the quadtree, I would like to implement the quadtree myself because I I still haven't chosen an interface. I want everything related to litecanvas to be lightweight and easy to minify.