Added getAABB() methods to Polygon and Circle

With this commit, you can get axis-aligned bounding boxes (AABB) for Polygons and Circles

The `getAABB()` method returns a Box, now. Do you prefer a Polygon?
And is the name `getAABB` OK?
Minor code style thing - can you put a space between the `for` (and `if`) and the opening `(`?