Add an ICollisionCallback to ISceneNodeAnimatorCollisionResponse to inform the application when collisions occur (and allow it to ignore them).  Thanks to garrit for this patch!  The existing collisionResponseAnimator unit test has been updated to exercise the new functionality.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@2111 dfc29bdd-3216-0410-991c-e03cc46cb475