Feature_copter extra channels mapping

This adds parameters to copter for channels 5,6,7,8

The Parameter names are discussable.
Let me know and i'll change them to whatever you like.

Also there is a warning for changing RCMAP_THROTTLE since this can cause problems with internal PPM Encoder on APM

This resolves:
https://github.com/diydrones/ardupilot/issues/1096
https://github.com/diydrones/ardupilot/pull/1103
https://github.com/diydrones/ardupilot/issues/685
Heli have CH_ROTORSPEED instead of CH_AUX2