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?
This method will return the AABB of the polygon _before_ any calculated translations/rotations are applied.

E.g if I do:

```
var myPoly = new Box(new Vector(0,0), 100, 100).toPolygon();
myPoly.setAngle(Math.PI / 4);
var myAABB = myPoly.getAABB();
```

It will return the bounding box for the original box, not the diamond resulting from the rotation. (It will work if `rotate` is called instead of `setAngle`)

I think if you change this to use `calcPoints` rather than `points` it will always return the AABB of a snapshot of the current state of the `Polygon`, which I think is a more predictable behavior for the method.