Fixed up the MotorJoint

Don't crash when setting linearOffset before joint creation.

Also, when linearOffset and angularOffset are set before joint creation,
their values now actually take effect whereas before they were ignored.
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)
Fixed up the RopeJoint

* Don't crash when initialized before creation
* Added defaulting to center of mass for local anchors
* Added warning when maxLength is too small (according to Box2D's
  documentation)
Fixed up the WeldJoint

* Don't crash when initialized before creation
* Separated defaulting for localAnchorA, localAnchorB and referenceAngle
Fixed up the WheelJoint

* Don't crash when initialized before creation
* Separated defaulting for localAnchorA, localAnchorB and localAxisA
* Added inverting of Y axis for localAxisA and normalized it, since
  non-normalized values were doing strange things.