Separate timer code from PWM/PPM code. The method of looking up a callback is now generic which means other (forthcoming) code can configure callback handlers.
Killed some leftovers of GPIO_Pin* stuff I forgot about.

git-svn-id: https://afrodevices.googlecode.com/svn/trunk/baseflight@387 7c89a4a9-59b9-e629-4cfe-3a2d53b20e61