ENH: Throttle PID Attenuation

Addresses #1160 based on [baseflight code](https://code.google.com/p/afrodevices/source/browse/trunk/baseflight/src/mw.c#100). Please review / comment, I'm not sure if this is the most optimal way to implement this.
So does the other code actually apply a uniform scaling across  all of the components? In their code does it affect PID constants or are they in units of time for the integral and derivative?