Revamp `media.py`

[ebb-earl-co]

This pull request aims to bring in a necessary bug fix: #4 , as well as some code cleanup in media.py:

• Make media.Track.get() take album and metadata arguments so that media.Album.get_tracks() can directly call media.Track.get()
• If only 1 URL is presented by the manifest, GET it with HTTP range requests
• Supporting the above, in requesting.py add three functions that generate range request headers for a given URL

[ebb-earl-co]

Good to close #4 with this PR.