Give Bitrate copy semantics.

For such a simple type, which will likely never change
to be un-copyable, it's nicer to have copy semantics.

src/session.rs

@@ -28,6 +28,7 @@ use version;
 
 use stream;
 
+#[derive(Clone, Copy)]
 pub enum Bitrate {
     Bitrate96,
     Bitrate160,