Make Torrentor handle magnet: links

Hi, I have another small one. :)

This patch makes Torrentor handle magnet: links directly -- theoretically. WebPositive, however, doesn't recognize them as links and tries to open HTTP://magnet:something. It's a bug in Web+.

You can test it though by copying the link and running in a terminal:
~> open "magnet:..."

I don't know where one can file bugs for Web+ but if they fix that, there is a pretty nice torrent "workflow" on Haiku, thanks to you! Thanks for writing Torrentor, and thanks for releasing the source code!

Hopefully I can contribute a few other patches in the future (if you're interested).