RigidBodyPropellor.m

Adding RigidBodyPropellor.m. This adds both a thrust and a moment due to aerodynamic drag from a propellor spinning, allowing a quadrotor to directly control its yaw.
build server pointed out that you need something here.  but it's better to use
`addpath(fullfile(pwd,'..'));`

(also, we will soon have a nice way to addpath just for the duration of the file, after Robin finishes his pull request)