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 originally changed it back in okready/three.js@b57d981e13aeb24172ba62ec278ba0d9690432df. It mainly makes a difference if you're using an uncompressed RGB565 or DXT-compressed texture, as both provide an extra bit of precision in the green channel versus the red or blue channels (DXT stores colors in its lookup table as RGB565 for each block).