Files storage: implement ability fto store public URLs without http / https prefix (only //)

[Audrunas]

Instead of http:// or https://, files may be stored only with // Test with all the storage engines: file storage, ftp, azure (secured / unsecured), amazon (secured / unsecured), azure/amazon with cloud storage

http://www.bettercms.com/support/bugs-and-suggestions/preview-image-url-doesnt-honor/

clafollett: I would at least recommend using "//" instead of "http://" or "https://". At least the URLs can run in mixed mode without issue.