Fix crossOrigin parameter for obj/mtl loaders

Adds or fixes the crossOrigin parameter in the OBJ/MTL loaders.
I added it to the ```OBJLoader``` because it was already referenced: https://github.com/mrdoob/three.js/blob/master/examples/js/loaders/OBJLoader.js#L20

Do you dislike it on ```OBJMTLLoader``` also? Without it it will be impossible to load textured models across domains. We could remove it as a parameter and put it into the options object?