Minor enhancements

• Added random_band(). Used mainly for testing, but may be useful for some random exploration.
• Better handling of empty values, like "Unknown", "N/A", ["N/A"], "" etc. Now all such data is returned as None or [] (empty list).
• Change repr and str for EntityArtist subclasses to use names from specific entities, not from underlying Artist. This is to avoid fetching Artist page just to display name, repr or str.
• Minor fixes
• Split code into smaller modules