Backend work for implementing zip download of compilations.
Adds a require 'zip/zip' line to the RubyZip gem.  Gives us access to Zip::ZipOutputStream
Moves the download action above the resources :compilations call, so that it gets matched.
Adds the download compilation link.
Implements a job that zips the entries in a compilation and puts them in the /tmp folder.  Not quite as far as sending the file back to the user yet.