Optimize image copy in ShopImport module

2 things :
- copy images to new folder structure instead of legacy one since it's a PS 1.5 branch
- don't create all sizes of images at migration, there's an Admin controller for that (Admin > Settings > Images). That's speed up the import and remove duplicate code (less code to support).

+ some cleaning of unused variables
Merge pull request #699 from sjousse/patch-1

[*] MO : ShopImporter Optimize image copy in ShopImport module