v0.5.1

Minor cleanup, tracking upstream changes.

• Added header_list! macro variant to simplify encoding Headers. Invoked as header_list![ "key1" => "value1", "key2" => "value2", ... ]
• Modified Session::send and Session::receive to return an IoResult.
• std::io renamed to std::old_io as Rust developers refactor std::io.
• #[allow(unstable)] dropped in favor of per-feature allow statements