Multirotor attitude controller updated

 - multirotor_att_control: performance significantly improved (tested on 3DR Quad frame), using `kD * (rate_prev - rate)` for D component of rate loop. Logs shows clean output without any spikes.
 - PID library: new functionality and bugfixes.

Users must be noticed about changes, MC_ATTRATE_D must be tuned. It will be great if somebody else tests this on multirotors and fixedwing before merge.