Parse #EXT-X-MEDIA in variant playlists.

The data is available in two places:

 - M3U8.media is a list of all Media entries.
 - Playlist.media is a list of the Media entries connected
   to this playlist via the AUDIO, VIDEO and SUBTITLES tags.
Update tests to support media in variant playlists.
Update README.rst.
Merge pull request #28 from chrippa/variant_playlist_media

Parse #EXT-X-MEDIA in variant playlists.