Move Song-related constants into models/song.rb
Song: Set @id before @filename so it's first item in the object
Move mp3 mocking into test_helper.rb and add support for unit tests
Move refresh() from SongsController to Song
Add unit test file for Song
Song: Add .all and .all_as_json
Song: Add .find
Arrange SongTest tests alphabetically
SongsController: Use Song model's .refresh and .all_as_json
SongsController: Remove obsolete "require 'logger'"