support web seeds in magnet links

git-svn-id: http://libtorrent.googlecode.com/svn@9888 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
introduce more asserts in torrent::file_progress and fix test_threads

git-svn-id: http://libtorrent.googlecode.com/svn@9889 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda