Added magnet links support via cache services.

Also this fix issue https://github.com/hexafluoride/byteflood/issues/12

This commit add the ability to load magnet links by downloading a cached .torrent file from online torrent cache services such as http://torrage.com and http://torcache.net. Additional services can be added by implementing the `ITorrentCache` interface. This way I download a .torrent file in the default download directory, and load torrent normally by file.

I have not added the code to load torrent without files (pure magnets, no cached .torrent), I left it for you hex so you can do it the way you prefer.