Css class && preview type

Sorry for the identical comment in both commits. I was hoping to be able to squash them together but somehow it doesnt work as I hoped.

The first commit introduces two new fields in the photo model: css_class && preview_type. Css class can be used to provide additional css classes to each photo and preview_type defines how the preview for the photo in the gallery will be rendered (e.g. album, single, preview). All two options + the url option is configured in the initializer and set to false as default. To see the options in the admin photo page, set them to true (the fields in the database will be generated regardless, though).

The second commit just does a refactore on the config variables.

Again, sorry for the fucked up commits.