Carrierwave, Rails 3.2 and some more related cleanup (corrected)

attachment_fu is difficult to use with Rails 3.2+, because of the strict attr_accessible policy. It is incompatible with Rails 4.0.

Carrierwave allows to build a much simpler system, which will be easy to extend to fix #49.

Implements #208

(Note: this branch is based on a still unmerged cleanup branch, so don't merge this until the other is merged)

(note: work in progress - the upload forms are still missing)