- based on some ideas from this thread: http://www.multiwii.com/forum/viewtopic.php?f=8&t=3671
- the sticks scaling is no more affected by PID coefficients
- yaw rate (to the right of the PIDs in GUI) now works as stick scaling
- default yaw rate is increased (with yaw rate at 0)
- yaw PID principle is now different from PITCH&ROLL PID:
 - yaw ITerm windup is very high, allowing an 'elastic' direction return after a manual perturbation
 - yaw ITerm is also constrained with a windup independent limit 
 - yaw PTerm is constrained in order to counter the yaw jump effect (maybe to refine)
 - yaw ITerm is canceled if the yaw stick is not centered

git-svn-id: http://multiwii.googlecode.com/svn/trunk@1474 02679b44-d973-9852-f2fa-63770883b36c