Initial adoption of closures in the math library.

I've implemented JavaScript closures across the whole math library to remove the need for declaration of public static variables for GC optimization purposes as proposed in issue #2936.

I did have to switch to extending the prototypes of the math classes rather than just replacing their prototypes.  I had to do this because instances of a type created within closures of that type are created before the prototype is set, thus they were incomplete types.  For example, if inside ```THREE.Matrix4.prototype = { ... [in here].... }``` you had a closure that created a ```var m1 = THREE.Matrix4()``` variable, it wouldn't yet have its prototype set because only after it has completed executing all the code in the ```{ ...[in here]... }``` block does it set it to ```Matrix4.prototype```.  By extending the prototype, even variables created inside the ```{...[in here]...}``` block eventually get full prototypes before they are used in their methods.

I added THREE.extends to handle extending prototypes, I based it on the code here: 

http://stackoverflow.com/a/12317051

I tested about 20 examples at random and they all worked.  All unit tests pass as well.

Both ```webgl_performance.html``` and ```webgl_performance_static.html``` perform at the same speed, no slowdowns on my PC.