Add cpShapeIsPoly, cpShapeIsSegment, cpShapeIsCircle functions

[mcclure]

Addresses #219.

Imagine you are writing a cpShapeEach callback to draw a scene which contains both circle and poly shapes. At the moment, there is no way to determine whether any one shape addressed by the callback is a circle, a poly, or something else. However telling circles and polys apart is necessary because functions like cpPolyShapeGetCount() assert-crash when called on the wrong kind of shape.

This patch adds three new functions cpShapeIsPoly, cpShapeIsSegment, cpShapeIsCircle that do the expected thing. I am indifferent to whether this is the best naming for these functions.

I tested this by doing a successful build and calling cpShapeIsPoly in live code (although my tests were in a feature branch based on older Chipmunk.) I did not attempt to modify any documentation.