AP_Mission MIS_AUTORESET parameter, methods

Minor additions to help pause/continue mission functionality (for #349)

Currently plane defaults to continuing missions between AUTO invocations, while copter defaults to resetting each AUTO invocation. Copter will change to match plane with AC3.2, but we need to provide a parameter for people to get the old behaviour if they prefer, which some plane users might also like too.

- added MIS_AUTORESET parameter
- added start_or_resume() method, calling either start() or resume() based on MIS_AUTORESET
- added reset() method to be able to reset mission state while not in AUTO