Refs #63, posts must have riff audio.

I have removed the Riff model entirely, and its fields have been added to the Post model. The  table in the database has been removed. Some changes were needed in the MediaFiles model, since the audio was using a riff_id where now it is using a post_id. The post generating functions in the test environments needed to be changed. Post::add() had its interface changed, since required and optional parameters got shifted around.

Instead of using the deprecated @filename syntax, uploading files via cURL now uses the CURLFile class.