Update URDF Importer to process sensor elements.

Update the URDF Importer to add sensors as 0 mass 0 inertia elements
attached by a fixed joint. This way their states can be read as links.

examples/Importers/ImportURDFDemo/URDF2Bullet.h

@@ -31,6 +31,7 @@ enum ConvertURDFFlags
 	CUF_ENABLE_SLEEPING = 2048,
 	CUF_INITIALIZE_SAT_FEATURES = 4096,
 	CUF_USE_SELF_COLLISION_INCLUDE_PARENT = 8192,
+  CUF_PARSE_SENSORS = 16384,
 };
 
 struct UrdfVisualShapeCache