Adding the classmethod from_string in the Torrent class, which allows to

build a Torrent object from its url. Fixes issue #65.

I added beautifulsoup as a new dependency. I'm sorry, I just can't stand lxml. It might also not be be the smartest way to do the job, but the job is done.
Yeah, I know it's not ok in python 2...What can I do instead ?