Added AB_OmniDrive() for 4 wheeled holonomic movement.

Also added AB_SetRotationPower(int), which is applied to holonomic
rotation.
Changed movement function paramaters from (y, x) to (x, y).

Also added AB_ScaleJoystick(joystick.joy1_y1) to easilly scale a joystick
value (-127 to 127) to (-100 to 100) for use in AB movement functions.