Keep track of a direction-of-movement on Transform

This vector will be used to determine where the given Transform needs to
move for the next frame.
Support multiplication of Vectors with scalars and other Vectors

Because we can't overload methods in Go, I've named the methods Scale,
for multiplcation with a Scalar, and Times, for multiplcation with
	another Vector.
Add Transform Behavior which moves Entities around by MoveDir and Speed

Updated the textured cube scene to allow moving the box around with ESDF
keys.
Fix Vector.Normalize to work with 0 length Vectors

Divide by 0 is a NaN, that doesn't work too well with the rest of the
system.