Add icon scaling for Growl

Also use Python's standard logger so we can support optional debug messages.
Make sure we do not try to convert None
Swap load_image_(gntp|legacy) around
Update README to reflect general dependency on Pillow/PIL
Add support for songs added with full path
Nitpick DBUS to D-Bus
Fix default image not being used on retry when there is no album art
Add "folder.jpg/png" as potential album art image
Replace python-mpd with python-mpd2 in readme