Add support for mp3 outputs

This adds an additional mp3 output to the Zencoder transcoding job which gets created for lessons that are being archived for download.

It also adds "mp3" to the set of `util.DOWNLOADABLE_FORMATS`, which is used when determining whether or not an output format exists for a given youtube-id.

**Note:** I was unable to run this in a good testing environment, so merge with caution! However, it *looks* like it will work. :+1: