Add option to zip galleries

When `zip_gallery` is True, each original file will be added to an archive named
`zip_name` located in the gallery directory.
Set zip_gallery to TRUE only if archive exists
Use single setting name
Add test cases for zipping galleries
Fix breadcumb -> breadcrumb
Merge pull request #24 from matze/fix-breadcrumb

Fix breadcumb -> breadcrumb
Remove obsolete setting
Merge pull request #23 from matze/zip-galleries

Add option to zip galleries