Allows multiple derivative types.

Hi,

For a project, I need other derivative types than "fullsize", "thumbnail" and "square_thumbnail". In particular, I need mini square thumbnails to build a pretty view and another type between "fullsize" and "original" for a site with big images.

So this little patch allows that. A plugin example is available at https://github.com/omeka/Omeka/pull/638.

Sincerely,

Daniel Berthereau
Infodoc & Knowledge management