ArduCopter arm/disarm support in mavutil


The ArduCopter trunk now supports arming and disarming motors via MAVLink message.  These changes to add the methods `arducopter_arm(self)` and `arducopter_disarm(self)` to the `mavfile` class in mavutil.py, which sends an appropriate `COMMAND_LONG` message.

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For reference, commit message to the ardupilot-mega repository for implementing the arm and disarm commands in ArduCopter/GCS_Mavlink.pde:

commit 83c7f035b551a351760ff26dbd5a0ec4fc92a424
Author: Pat Hickey <pat@moreproductive.org>
Date:   Tue Oct 9 18:48:05 2012 -0700

    ArduCopter GCS_MAVLink: COMMAND_LONG for arm/disarm motors
    
    Date: Wed, 26 Sep 2012 15:56:43 -0700
    Subject: ArduCopter arm/disarm command consensus
    From: Pat Hickey <pat@moreproductive.org>
    To: Michael Oborne <michael@oborne.me>
    Cc: "Craig J. Elder" <craig@3drobotics.com>, arducopter
    <arducopter@googlegroups.com>,
        mavelous <mavelous@googlegroups.com>
    
        Michael,
    
        Per our discussion today,
    
        In a MAVLINK_MSG_ID_COMMAND_LONG
        A MAV_CMD of type MAV_CMD_COMPONENT_ARM_DISARM
        with component id MAV_COMP_ID_SYSTEM_CONTROL = 250,
        uses param index 1 to specify an arm/disarm motors event: 1 to arm,
        0 to disarm
    
        Thanks for working this out with me. Sorry to get it so completely
        wrong the first time around!
    
        Best
        Pat