ManualControl: add an expo parameter for rate mode

The implementation is based on linearly interpolation from 0 to
100% between x and x^3 of the input (-1,1) and does not change the
rate for the extreme sticks.  Based on discussion here

http://www.rcgroups.com/forums/showthread.php?t=452369&page=3
Merge pull request #872 from TauLabs/rate_expo

ManualControl: add an expo parameter for rate mode