Add method to control vehicle using Velocity

[hamishwillee]

Velocity control is a useful and common method of control in Copter - so we should provide access through the API.

See Velocity control for method that might be used.

This command makes no sense in Plane/Rover so we need a strategy for what we do if connected to those vehicles.

[hamishwillee]

@tcr3dr #226 is IMO more important than this issue, because it will never be possible to use velocity control on Plane.