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.
Moved "alphamap_fragment" shader chunks immediately after "map_fragment".

This ensures proper alpha testing and avoids other potential issues with deferring alpha map blending until late in the fragment shader.
Use the green channel from alpha maps instead of red.

More precision is used for the green channel in RGB565 and DXT texture formats, so this would improve the quality in those cases.
Merge branch 'dev' into patch-3

Conflicts:
	src/renderers/shaders/ShaderChunk.js
Added alpha map fragment shader sources to the build includes.
Updated documentation for MeshBasicMaterial, MeshLambertMaterial, and MeshPhongMaterial to include information about the alphaMap property.
Merge pull request #4987 from okready/patch-3

Support for separate alpha map textures.