Issue/attachments show not download

* Adds method download_url to attachment
* http://stackoverflow.com/questions/12939903/allowing-user-to-download-file-from-s3-storage#answer-16426114
* Fix for #291 
* Much cleanup so PR looks like a mess

for actual changes see commit: https://github.com/ophrescue/RescueRails/commit/670164e50d55cb6739a18293f2f33a0e9e13bac4