Add support for category tabs in 3D view's tool shelf, define new "MMD" category.
Merge pull request #19 from iRi-E/master

Add support for category tabs in 3D view's tool shelf.
Fix button layout of the panel.
PMX importer: Don't use alpha channel of sphere texture.
Follow up to previous commit: Avoid error when failing to load sphere texture.
Avoid error that occurs in Blender built without OpenColorIO.
Merge pull request #20 from iRi-E/fix_issues

Fix some issues