expose the _measured_ MyAvatar.getVelocity() to JS

This project is an attempt to make MyAvatar.getVelocity() work properly for JS scripts that want to use it.  There were a couple of problems:

(1) MyAvatar.getVelocity() was always returning <0,0,0>.  It now returns the _measured_ velocity of the avatar after all effects (after collisions, friction, etc).

(2) Once issue (1) was fixed the value returned could vary (with incorrect values) depending on when the script actually captured the the velocity in the main simulation loop.  Granted, the window for incorrect values was probably pretty narrow but nevertheless non-zero.  Progress has been made (but is not perfect) to isolate the measured velocity from intermediate calculations.
This stuff was moved into MyAvatar::updatePosition() from three methods that used to be separate:

MyAvatar::updateMotorFromKeyboard()
MyAvatar::applyMotor()
MyAvatar::updateThrust()

We used to store temporary values in MyAvatar::_velocity which would then be modified by these and other code before it was finally applied (below).  We now keep a variable called "velocity" in with local scope to MyAvatar::updatePosition() which starts with the value of _velocity and is modified by various features until it is finally applied.  After that the real _velocity is measured in Avatr::measureMotionDerivatives().