AP_Mission: Command and waypoint handling library.

- Reads and writes MAVLINK commands to/from storage.
- Contains a waypoint queue of previous, current, and after waypoints.
- Provides commands associated with current mission leg.
- Allows incrementing waypoints and changing to specific waypoints at anytime.
- Handles DO_JUMP commands while maintaining a valid waypoint queue.
- Performs error checking on waypoint values.