implemented rolling friction, using a contact constraint. Useful to get rolling spheres to rest, even on a slightly sloped plane.

See http://www.youtube.com/watch?v=RV7sBAsKu4M and Bullet/Demos/RollingFrictionDemo
Fixes in FractureDemo (mouse picking constraint needs to be removed, otherwise constraint solver crashes/asserts)

src/BulletCollision/CollisionDispatch/btCollisionObject.h

@@ -85,6 +85,7 @@ protected:
 
 	btScalar		m_friction;
 	btScalar		m_restitution;
+	btScalar		m_rollingFriction;
 
 	///m_internalType is reserved to distinguish Bullet's btCollisionObject, btRigidBody, btSoftBody, btGhostObject etc.
 	///do not assign your own m_internalType unless you write a new dynamics object class.
@@ -263,6 +264,16 @@ public:
 		return m_friction;
 	}
 
+	void	setRollingFriction(btScalar frict)
+	{
+		m_rollingFriction = frict;
+	}
+	btScalar	getRollingFriction() const
+	{
+		return m_rollingFriction;
+	}
+
+
 	///reserved for Bullet internal usage
 	int	getInternalType() const
 	{