Added btGeneric6DofSpringConstraint and linear/angular limits to btGeneric6DofConstraint.
btGeneric6DofConstraint implements btTypedConstraint
Added getNumConstraintRefs and getConstraintRef to btRigidBody, removeAction to btDynamicsWorld and getCurrentSpeedKmHour to btRaycastVehicle.
Added btGeneric6DofSpringConstraint and linear/angular limits to btGeneric6DofConstraint.
btGeneric6DofConstraint implements btTypedConstraint
Merge branch 'improved-raycastvehicle' of https://github.com/kuang11/ammo.js into improved-raycastvehicle
Removed NO_DYNAMIC_EXECUTION to make JsImplemented classes work.
Minor tweaks.