message: refactor(lib): use try_nb! from tokio_io, not tokio_core
  tokio_core::io is deprecated, and the try_nb! conceptually belongs to
  that module. It is also available in tokio_io, so use that instead.
diff: @@ -23,8 +23,8 @@ extern crate language_tags;
  #[macro_use] pub extern crate mime;
  extern crate base64;
  extern crate time;
  -#[macro_use] extern crate tokio_core as tokio;
  -extern crate tokio_io;
  +extern crate tokio_core as tokio;
  +#[macro_use] extern crate tokio_io;
  extern crate tokio_proto;
  extern crate tokio_service;
  extern crate unicase;
  