Adjusted jukebox example to handle PlaylistFolder objects

I got a TypeError with the 'ls'/'list' command. This is a quick fix to get the example running.

```python
jukebox> ls
Stopping
Goodbye!
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 552, in __bootstrap_inner
    self.run()
  File "jukebox.py", line 35, in run
    self.cmdloop()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/cmd.py", line 142, in cmdloop
    stop = self.onecmd(line)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/cmd.py", line 219, in onecmd
    return func(arg)
  File "jukebox.py", line 54, in do_list
    if Link.from_playlist(p).type() == Link.LINK_STARRED:
TypeError: must be spotify.Playlist, not spotify.PlaylistFolder

Logged out!
```