Multiwii rate

This extends #1280 and implements a rate controller that is similar to that in baseflight/multiwii.
Specifically it adds the stick input to the output and then has gyro damping. The gyro damping
can be reduced with more stick (as determined by MWRollPitchRate and MWYawRate). 

As documented in the code, to convert from MW tuning settings to the matching ones here use

 - Kp = Kp_mw * 4 / 80 / 500
 - Kd = Kd_mw * looptime * 1e-6 * 4 * 3 / 32 / 500
 - Ki = Ki_mw * 4 / 125 / 64 / (looptime * 1e-6) / 500

Currently these settings are just dropped into the rate loop PID settings, but they will be split
out before merging.