Support for separate alpha map textures.

Added support for using a separate alpha map with MeshBasicMaterial, MeshLambertMaterial, and MeshPhongMaterial, and updated the OBJ converter to set materials as transparent if an alpha map is defined.
I'd say grayscale. You could technically put other data into the other channels, but that can cause visual artifacts when sampling from an individual channel if you're using a DXT-compressed format.

I've typically used the green channel in the past when sampling from grayscale textures, since we've tended to compress everything using DXT. Others recommend this as well (http://udn.epicgames.com/Three/TextureOptimizationTechniques.html, http://forum.unity3d.com/threads/best-texture-format-for-a-single-channel-mask-type-texture.205651/, and http://www.bencloward.com/tutorials_normal_maps12.shtml are a few pages I found that make a mention of this).