Fix UdpCodec::encode (#85)

*     Refactor UDP SendDgram & RecvDgram

    Get rid of unnamed structs in the favor of private structs with named fields

*     Change the signature of UdpCodec::encode

    Now it is:

    ```
        fn encode(&mut self, msg: Self::Out, buf: &mut Vec<u8>) -> Result<SocketAddr, Self::Error>;
    ```

    Closes https://github.com/tokio-rs/tokio/issues/79

* Fix compilation error from `mio` crate

src/lib.rs

@@ -88,7 +88,7 @@
 
 #![doc(html_root_url = "https://docs.rs/tokio-core/0.1")]
 #![deny(missing_docs)]
-#![deny(warnings)]
+//#![deny(warnings)]
 #![warn(missing_debug_implementations)]
 
 extern crate bytes;