~ Add docs to Torrent module.
~ Document bitfield.
~ Rename package to bittorrent.

It seems like 'network-bittorrent' is too long and annoying: everyone
knows that bittorrent is a network protocol and there is no purpose to
prefix it with 'network'. Moreover hackage do not have package with
the our new name.

So I think just 'bittorrent' is a more suitable name for the package.
~ Rename cabal file.
~ Update readme.