[issue-2] Fix localized images path and files structure

Localized images should be located in styles/{style_name}/theme/{iso}/ folder
rather than in styles/{style_name}/imageset/{iso}/ for Ascraeus
since there's no imageset in phpBB 3.1.x.
Add index.htm (optional) for all folders.
Merge pull request #3 from rxu/issue/2

[issue-2] Fix localized images path