Build type information on first use (load-time), instead of using a hardcoded list

--HG--
branch : dynamiccore2

src/modules/physics/box2d/PrismaticJoint.cpp

@@ -32,6 +32,8 @@ namespace physics
 namespace box2d
 {
 
+love::Type PrismaticJoint::type(&Joint::type);
+
 PrismaticJoint::PrismaticJoint(Body *body1, Body *body2, float xA, float yA, float xB, float yB, float ax, float ay, bool collideConnected)
 	: Joint(body1, body2)
 	, joint(NULL)