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.
Hmm.. looking at the baseflight code again, it seems like they only scale Kp and Kd. "dynI8" is computed but never used. The baseflight code doesn't use a "dT" and the PID constants are scaled directly. If I'm not mistaken, this means the PID constants in baseflight are sampling rate dependent, but I guess since they only support a small number of targets, which presumably all use the same sampling rate, it doesn't matter. Anyways, for a given "dT", scaling the PID constants by a percentage should do the right thing.