Refactoring to enable multi SAP broadphase.

This was already planned, and Pierre Terdiman recent thread motivated me to decouple the paircache from the sweep and prune.
http://continuousphysics.com/Bullet/phpBB2/viewtopic.php?t=1329

Demos/ColladaDemo/ColladaDemo.cpp

@@ -173,7 +173,7 @@ void	ColladaDemo::initPhysics(const char* filename)
 	btCollisionDispatcher* dispatcher = new btCollisionDispatcher();
 	btVector3 worldMin(-1000,-1000,-1000);
 	btVector3 worldMax(1000,1000,1000);
-	btOverlappingPairCache* pairCache = new btAxisSweep3(worldMin,worldMax);
+	btBroadphaseInterface* pairCache = new btAxisSweep3(worldMin,worldMax);
 	btConstraintSolver* constraintSolver = new btSequentialImpulseConstraintSolver();
 	m_dynamicsWorld = new btDiscreteDynamicsWorld(dispatcher,pairCache,constraintSolver);