added URL link frames for specific purposes for ID3v24

Hi,

I needed to use one of the URL link frames with a specific purpose declared within the ID3v2 spec version 4.0.
The frames start with W with three trailing characters (W000 - WZZZ) excluding WXXX which is reserved for user specific URLs and already implemented in your library.

To provide a complete patch, I implemented methods for all W-frames specified as of yet.
I hope it will prove useful for others.

Regards
cyc1ingsir