add move.js demo MyAvatar.motorVelocity feature

The avatar can now be moved around by specifying a "motorVelocity" via JS (previous PR). This PR adds a demo script that uses the new feature.
I think these two functions are essentially identical, except you changed the order of the parameters...

Seems like you should change your script to use the existing function and not add this new one.

```
 glm::vec3 Vec3::multiply(const glm::vec3& v1, float f) {
     return v1 * f;
 }
 
+glm::vec3 Vec3::multiply(float f, const glm::vec3& v1) {
+    return v1 * f;
+}

```