[chipmunk] Space.AddPostStepCallback
[chipmunk] equality comparers

implement ==, !=, Equals and GetHashCode on ChipmunkObject. Seal
Arbiter, Body, Constraints, Shapes and Space to avoid issues in user
code. Really need inheritance ? Think twice, and compose.
[chipmunk] Fix the callback passed to native

The callback used to work... on the simulator, but failed when I tried
them on a device. This adds a nice layer of abstraction to hide the fact
that you have to flag the callbacks with [MonoPInvokeCallback] and that
you can't have instance methods.
Merge pull request #106 from StephaneDelcroix/chipmunk

Chipmunk: AddPostStepCallback, equality overrides and fix the callbacks