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.
Ok. I can see where you're going with this. I wish we could add true operators to the scripting language. That would be preferable. 

But I can see how we might want these as a convenience. I just assumed you didn't see the other version when implementing your script. Since other that the order for convenience we agree these do the same thing. 

> On Sep 21, 2014, at 3:47 PM, AndrewMeadows <notifications@github.com> wrote:
> 
> In libraries/script-engine/src/Vec3.cpp:
> 
> > @@ -25,6 +25,10 @@ glm::vec3 Vec3::multiply(const glm::vec3& v1, float f) {
> >      return v1 * f;
> >  }
> >  
> > +glm::vec3 Vec3::multiply(float f, const glm::vec3& v1) {
> I disagree. Multiplication commutes so the order of the operands should not matter.
> 
> The alternative is an unexpected reality that violates the commutative property of multiplication. If you happen to use the wrong order you get NaN's injected into your math causing much confusion and frustration. In the end, when the reason for the problem is discovered there is cursing and gnashing or teeth. Ask me how I know this.
> 
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