AP_Mission: Library to handle auto missions and waypoints

AP_Mission: Fixed parameter naming.
Plane: integration with AP_Mission
AP_Mission: Addition of function to return home altitude only.
Plane: Removal of leftover g.command_total variables, transitioned to AP_Mission, mission.command_total()
Plane: When out of commands, transition to RTL, rather than staying in a redundant "RTL-like" state, auto with waypoint 0 as the active waypoint.  This simplifies the user interface, as there is only 1 RTL state.
Plane: Transitioned storage of home location to AP_Mission
Plane: Removed unused home_is_set variable.
AP_Mission: The old command_index() is now called waypoint_index().  Command_index() now returns the current "non-nav command"
Plane: Renamed mission.command_index() to mission.waypoint_index().  Removed nav_command_index and nonnav_command_index variables.  This functionality now resides in AP_Mission.
Plane: Added mission.init_commands()
AP_Mission: Fixed library to handle case where the first waypoint in the mission is not a navigation command.
AP_Mission: Updated test suite
Plane: Remove #define CMD_BLANK 0, use pre-existing #define NO_COMMAND 0.
Plane: Removed redundant variable, nav_command_ID, replaced with next_nav_command.id.
Plane: Removed check for a waypoint index of 0 during switch between modes.  This check is done by AP_Mission instead.
AP_Mission: Added mission status flag to provide persistent feedback on state of mission.
Revert "Plane: Removed check for a waypoint index of 0 during switch between modes.  This check is done by AP_Mission instead."

This reverts commit a27483101d8d0ac842c4797772a14954b30c0037.
Plane: Use the AP_Mission status flag when transitioning into auto mode mid-flight.  If the mission status is false (invalid/complete) the aircraft will enter RTL mode.  If the mission is complete, the pilot may send a change waypoint command or restart mission to begin again.
Plane: Removed old unused reload_commands_airstart() function
Plane: added struct in a couple places.