Rename torrent package to metainfo; expose MetaInfo["info"] so it can be used on its own

--HG--
rename : torrent/README => metainfo/README
rename : torrent/_testdata/archlinux-2011.08.19-netinstall-i686.iso.torrent => metainfo/_testdata/archlinux-2011.08.19-netinstall-i686.iso.torrent
rename : torrent/_testdata/continuum.torrent => metainfo/_testdata/continuum.torrent
rename : torrent/builder.go => metainfo/builder.go
rename : torrent/metainfo.go => metainfo/metainfo.go
rename : torrent/metainfo_test.go => metainfo/metainfo_test.go