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.

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

This is not tested