smoother avatar motion at low velocities

DaveDub says that the glitches in this velocity plot are causing problems for making procedural walk animations:

http://highfidelity.davedub.co.uk/images/avatarStateTransitionSpeedglitch.png

His test script is here:  http://highfidelity.davedub.co.uk/scripts/speedGlitch.js

I tweaked things to provide a smoother (and steeper) transition when the avatar starts off.  We can play with it some more but this should make it easier for walk animations on takeoff while also making it easier to navigate at low velocities and close quarters (when trying to get face to face with another avatar for example).