prefer 'processed' media over 'original' media when rendering graphic

This addresses #41.  Basically I added a two new scopes to the media model-- 'processed' and 'preferred'.  The 'preferred' scope is a combination of 'original' and 'processed' entries ordered so that the 'processed' entries are first.  This provides for minimal code changes throughout the rest of the application, simply changing `media.original.first` to `media.preferred.first` and if there is a processed entry it will use it otherwise it will use the original entry.