refactor(headers): add header macros and add two example uses

Add `impl_header!` and `impl_list_header!` macros. Rewrite `Accept-Encoding`
and `Server` header implementations to use the new macros.

src/header/common/server.rs

@@ -1,6 +1,4 @@
-use header::{Header, HeaderFormat};
-use std::fmt;
-use header::shared::util::from_one_raw_str;
+use header;
 
 /// The `Server` header field.
 ///
@@ -8,22 +6,8 @@ use header::shared::util::from_one_raw_str;
 #[derive(Clone, PartialEq, Show)]
 pub struct Server(pub String);
 
-deref!(Server => String);
-
-impl Header for Server {
-    fn header_name(_: Option<Server>) -> &'static str {
-        "Server"
-    }
-
-    fn parse_header(raw: &[Vec<u8>]) -> Option<Server> {
-        from_one_raw_str(raw).map(|s| Server(s))
-    }
-}
-
-impl HeaderFormat for Server {
-    fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        fmt.write_str(&*self.0)
-    }
-}
+impl_header!(Server,
+             "Server",
+             String);
 
 bench_header!(bench, Server, { vec![b"Some String".to_vec()] });