use consistently `conn' as local variable name

Having http::error being io::error plus badrequest was just a hack. There's no need to expose the private error type.

since it uses a bufio::stream it is limited to 1024 bytes of response, an auto-resizing implementation will follow...

HTTP/1.0 doesn't support chunking

Still very WIP and the current API will be subject to changes, but it's enough to be imported in the main tree.