Add tGeneric6DofSpring2Constraint support for btMultiBody

[erwincoumans]

Right now, btMultiBody has very limited support for constraints, mainly contact constraints,btMultiBodyPoint2Point and btMultiBodyJointMotor.

Implement support for btGeneric6DofSpring2Constraint, then we cover most cases.

[benelot]

This might be relevant in the same run:
#463 Implement btMultiBodyJointMotor and btMultiBodyJointLimitConstraint for Spherical Joint