Remove semicolon after import statement.
Arrange the python scripts under a common package.

The 8tracks client implementation is now available in the module eighttracks.api.
The command line interface of is available via the package
eighttracks.download.cli. The 8tracks.sh script can be used as a shortcut to
invoke the command line interface.
Separate 8tracks API client from download logic.
Return a Track object from Playlist.get_next_track.
Write track file as it is downloading.

Instead of saving the whole downloaded track in memory and writing it to the
disk afterwards, this commit writes the file while the download is running.
Fix ID3 tagging: Mixed up artist and song title.

The song name is tagged as the artist and the artist is tagged as the song name.
This is fixed by this commit.