~gems: Added carrierwave, carrierwave-mongoid and mini_magick for
dealing with file uploads.
Updating spots via the frontend now works as the location key which
is generically sent empty currently, is now discarded and thus can't
prevent the spot from updating.
Spot now has a CarrierWave uploader mounted on :photo.
Because the file is sent via jquery.iframe-transport we need to
accept an update request as POST.