Expose Codec via an unstable flag (#49)

Exposes `Codec` using an unstable flag. This is useful for testing.

examples/client.rs

@@ -2,12 +2,10 @@ extern crate h2;
 extern crate http;
 extern crate bytes;
 extern crate futures;
-extern crate tokio_io;
 extern crate tokio_core;
 extern crate io_dump;
 extern crate env_logger;
 
-use h2::*;
 use h2::client::{Client, Body};
 
 use http::*;