Added btGeneric6DofSpringConstraint, set/get breakingThrehsold to btTypedConstraint, btContactSolverInfo and two btWheelInfo attributes
Merge pull request #92 from Kuang11/master

Added btGeneric6DofSpringConstraint, set/get breakingThrehsold to btTypedConstraint, btContactSolverInfo and two btWheelInfo attributes.