Added btMultiBodyPoint2Point, it can be used between btMultiBody vs btMultiBody or btMultiBody vs btRigidBody

Allow picking of btMultiBody, using a btMultiBodyPoint2Point constraint, with limited strength to avoid adding too much energy to the system (= blowup)
Add btMultiBodyJointMotor, it can be used in combination with joint limit (just add the joint limit after the motor, to avoid jitter)

src/BulletDynamics/Featherstone/btMultiBodyJointMotor.h

@@ -0,0 +1,47 @@
+/*
+Bullet Continuous Collision Detection and Physics Library
+Copyright (c) 2013 Erwin Coumans  http://bulletphysics.org
+
+This software is provided 'as-is', without any express or implied warranty.
+In no event will the authors be held liable for any damages arising from the use of this software.
+Permission is granted to anyone to use this software for any purpose, 
+including commercial applications, and to alter it and redistribute it freely, 
+subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
+2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.
+*/
+
+///This file was written by Erwin Coumans
+
+#ifndef BT_MULTIBODY_JOINT_MOTOR_H
+#define BT_MULTIBODY_JOINT_MOTOR_H
+
+#include "btMultiBodyConstraint.h"
+struct btSolverInfo;
+
+class btMultiBodyJointMotor : public btMultiBodyConstraint
+{
+protected:
+
+	btScalar	m_maxMotorImpulse;
+	btScalar	m_desiredVelocity;
+
+public:
+
+	btMultiBodyJointMotor(btMultiBody* body, int link, btScalar desiredVelocity, btScalar maxMotorImpulse);
+	virtual ~btMultiBodyJointMotor();
+
+	virtual int getIslandIdA() const;
+	virtual int getIslandIdB() const;
+
+	virtual void createConstraintRows(btMultiBodyConstraintArray& constraintRows,
+		btMultiBodyJacobianData& data,
+		const btContactSolverInfo& infoGlobal);
+	
+	
+};
+
+#endif //BT_MULTIBODY_JOINT_MOTOR_H
+