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.cpp

@@ -0,0 +1,89 @@
+/*
+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
+
+#include "btMultiBodyJointMotor.h"
+#include "btMultiBody.h"
+#include "btMultiBodyLinkCollider.h"
+#include "BulletCollision/CollisionDispatch/btCollisionObject.h"
+
+
+btMultiBodyJointMotor::btMultiBodyJointMotor(btMultiBody* body, int link, btScalar desiredVelocity, btScalar maxMotorImpulse)
+	:btMultiBodyConstraint(body,body,link,link,1,true),
+	m_desiredVelocity(desiredVelocity),
+	m_maxMotorImpulse(maxMotorImpulse)
+{
+	// the data.m_jacobians never change, so may as well
+    // initialize them here
+        
+    // note: we rely on the fact that data.m_jacobians are
+    // always initialized to zero by the Constraint ctor
+
+    // row 0: the lower bound
+    jacobianA(0)[6 + link] = 1;
+}
+btMultiBodyJointMotor::~btMultiBodyJointMotor()
+{
+}
+
+int btMultiBodyJointMotor::getIslandIdA() const
+{
+	btMultiBodyLinkCollider* col = m_bodyA->getBaseCollider();
+	if (col)
+		return col->getIslandTag();
+	for (int i=0;i<m_bodyA->getNumLinks();i++)
+	{
+		if (m_bodyA->getLink(i).m_collider)
+			return m_bodyA->getLink(i).m_collider->getIslandTag();
+	}
+	return -1;
+}
+
+int btMultiBodyJointMotor::getIslandIdB() const
+{
+	btMultiBodyLinkCollider* col = m_bodyB->getBaseCollider();
+	if (col)
+		return col->getIslandTag();
+
+	for (int i=0;i<m_bodyB->getNumLinks();i++)
+	{
+		col = m_bodyB->getLink(i).m_collider;
+		if (col)
+			return col->getIslandTag();
+	}
+	return -1;
+}
+
+
+void btMultiBodyJointMotor::createConstraintRows(btMultiBodyConstraintArray& constraintRows,
+		btMultiBodyJacobianData& data,
+		const btContactSolverInfo& infoGlobal)
+{
+    // only positions need to be updated -- data.m_jacobians and force
+    // directions were set in the ctor and never change.
+    
+  
+
+	for (int row=0;row<getNumRows();row++)
+	{
+		btMultiBodySolverConstraint& constraintRow = constraintRows.expandNonInitializing();
+		
+		btScalar penetration = 0;
+		fillConstraintRowMultiBodyMultiBody(constraintRow,data,jacobianA(row),jacobianB(row),infoGlobal,m_desiredVelocity,-m_maxMotorImpulse,m_maxMotorImpulse);
+	}
+
+}
+