Fix some issues

1) PMX importer: Don't use alpha channel of sphere texture. (8721929, 6c29b8d)

Sphere texture should've used no alpha channel. Otherwise, if all pixels have alpha=0, the sphere texture takes no effect. The problem can be confirmed by importing Koron-shiki Miku model (http://www.nicovideo.jp/watch/sm22523244).

Note: If texture is .bmp file, "Use Alpha" checkbox doesn't appear in image texture UI though it actually works. I've fixed this bug upstream (https://developer.blender.org/rB6316306cc07c77d4cf6fff52bc8a35ba102b8f56).

2) Avoid error that occurs in Blender built without OpenColorIO. (c5d413f)

Using "View:Shadeless" button caused a TypeError if Blender was built without OpenColorIO. I used try-except rather than bpy.app.ocio.supported here, because bpy.app.ocio is available only in Blender 2.69 or later.