Fixed up the PrismaticJoint

* Don't crash when initialized before creation
* Added PrismaticJoint.referenceAngle
* Changed PrismaticJoint.axis to PrismaticJoint.localAxisA
Fixed up the PulleyJoint

* Don't crash when initialized before creation
* Added defaulting to center of mass for local anchors
* Added defaulting of lengthA/lengthB to the difference between local
  anchors and their respective ground anchors
Fixed up the RevoluteJoint

* Don't crash when initialized before creation
* Added RevoluteJoint.referenceAngle
* Fixed default behavior of localAnchorB in case localAnchorA is set
  to a value different from the center of mass of bodyA (also applied
  to FrictionJoint and PrismaticJoint)