This should fix doubleSided objects causing misbehaved shadows reported in #1758.

It's now done once per frame / plugin instead of per material.

Fixes #1757

That wasn't supposed to go in...

I tried to move BinaryLoader to /examples/js but SceneLoader needs it.
Curious to hear what people thinks about this change.

That was a tough one: spotlights were randomly working / not-working based on completely unrelated stuff.

This adds 2 extra parameters to SpotLight:

- "angle" for controlling light cone width
- "exponent" for controlling sideways light attenuation (from light->target axis towards edges of light cone)

Warning: this changes appearance of everything that used old fake SpotLight. To get approximately similar look use these parameter values:

```
spotLight.angle = Math.PI;
spotLight.exponent = 1;
```

Todo:

- implement spotlights in normal map and skin shaders
- check what's going on with vertex colors and ambient term in unlit parts
- check shadowmap darkening vs unlit parts darkening
- maybe some better formula for light cone attenuation, current one varies too wildly with cone angle

This reverts commit de6d62bf.

(Phewww!) I'm sure there are still things I missed...

Some weird warning saying THREE.REVISION isn't defined :/
Need to improve this build.py some day...

This is now done as method of Matrix3 object which gets as a parameter Matrix4 object to be inverted (to be consistent with how Matrix4 => Matrix4 inversion is done).

As per discussion in #1615.

Should fix issue #1647