Implement Vehicle.waypoint_home

[hamishwillee]

Vehicle.waypoint_home was in the original documentation, but is not present in the implementation (MPVehicle). According to @geeksville (#100)

this was probably inadvertently lost when full command access was added (.. vehicle.commands[0] is guaranteed to be home). It would be easy to add it back.

Action is to add an implementation for getting this value. Also to consider adding a setter - that makes sense to me because then you could easily change the home position outside of missions.

[hamishwillee]

@tcr3dr I am closing this as it is a "subset" of #252 (covers just the creation of the waypoint home, while 252 also covers hiding the waypoint from the commands list).