Don't unbind createMaterial in Loader

For some reason, `initMaterials` was calling `createMaterial` directly on `prototype`, instead of `this`, causing any changes to an individual loader to be ignored. Specifically, this was the problem:

```javascript
var loader = new THREE.JSONLoader();
loader.crossOrigin = 'anonymous';
loader.load(/*...*/);
```

On [this line](https://github.com/sgrif/three.js/blob/sg-cross-origin/src/loaders/Loader.js#L173), _this would still be referencing the prototype, ignoring the `crossOrigin` property that had been set on the loader.