Expose Codec via an unstable flag (#49)

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

src/lib.rs

@@ -1,4 +1,4 @@
-// #![deny(warnings, missing_debug_implementations)]
+#![deny(warnings, missing_debug_implementations)]
 
 #[macro_use]
 extern crate futures;
@@ -28,9 +28,17 @@ mod error;
 mod codec;
 mod hpack;
 mod proto;
+
+#[cfg(not(feature = "unstable"))]
 mod frame;
 
+#[cfg(feature = "unstable")]
+pub mod frame;
+
 pub mod client;
 pub mod server;
 
 pub use error::{Error, Reason};
+
+#[cfg(feature = "unstable")]
+pub use codec::{Codec, SendError, RecvError, UserError};