More GetReactionForce/Torque Fixes

[wnbittle]

I noticed a couple of other issues based on the PR to fix #634:

1. b2_revolute_joint.cpp GetReactionTorque has the wrong sign

- return inv_dt * (m_lowerImpulse + m_upperImpulse);
+ return inv_dt * (m_lowerImpulse - m_upperImpulse);

2. b2_prismatic_joint.cpp GetReactionForce has the wrong sign

- return inv_dt * (m_impulse.x * m_perp + (m_motorImpulse + m_lowerImpulse + m_upperImpulse) * m_axis);
+ return inv_dt * (m_impulse.x * m_perp + (m_motorImpulse + m_lowerImpulse - m_upperImpulse) * m_axis);

3. b2_distance_joint.cpp GetReactionForce isn't including the limit impulse

- b2Vec2 F = (inv_dt * m_impulse) * m_u;
+ b2Vec2 F = (inv_dt * m_impulse + m_lowerImpulse - m_upperImpulse) * m_u;

[thegrb93]

This PR? https://github.com/erincatto/box2d/pull/641

[erincatto]

Fixed in #653