Split codec code (#128)

* move src/codec.rs -> src/codec/mod.rs
* Move traits Encoder and Decoder from src/framed_{read|write}.rs into src/codec/{encoder,decoder}.rs
* Move LinesCodec and BytesCodec from src/codecs.rs into src/codec/{lines,bytes}_codec.rs

tokio-io/src/lib.rs

@@ -49,7 +49,6 @@ pub mod io;
 pub mod codec;
 
 mod allow_std;
-mod codecs;
 mod copy;
 mod flush;
 mod framed;