Audio refactor

This branch greatly simplifies the audio model.

* Removes the STI nonsense.
* All audio now only uses its URL attribute for finding audio.
* Program Audio and ENCO Audio are now just small modules which scrape the filesystem and create normal audio objects.
* Updated Audio model to use StatusBuilder.
* Removed all the old STI columns (enco_number, enco_date, type, path, store_dir, etc.)
* Added random string to uploaded mp3 files to ensure uniqueness.
* mp3 is no longer mounted with Carrierwave. Instead, the Uploader class is used directly.