Changed entire movement code to match tutorial

I found that the tutorial did a more elegant way (MAC) of doing it.  The
constraints are defined in a different class (which is seralized so you
can see it in Unity3D's inspector) and instead of using if statements to
check if we are past boundaries, it now uses the clamp function to make
sure it never goes past the xMin, xMax, etc. I'll go over the code with
you, Mike.