Add app to gemspec's files
Add app to gemspec's files

Images were moved to the app directory for Rails 4, but they weren't added to the packaged gem.

Should also fix #20
varyform/fancybox2-rails,kyparn/fancybox2-rails,varyform/fancybox2-rails,kyparn/fancybox2-rails,varyform/fancybox2-rails,kyparn/fancybox2-rails