refactor(headers): Introduce header!() macro, improve documentation

The new macro handles single value headers, list headers, and list
headers with at least one item.
It creates the item for the header and contains its documentation. The
new macro allows handling
more header cases in the future, it will also be possible to include
tests inside the macro.

BREAKING CHANGE: Removed impl_header!() and impl_list_header!() macros,
use new header!() macro.

src/header/common/accept_encoding.rs

@@ -1,15 +1,22 @@
 use header::{Encoding, QualityItem};
 
-/// The `Accept-Encoding` header
-///
-/// The `Accept-Encoding` header can be used by clients to indicate what
-/// response encodings they accept.
-#[derive(Clone, PartialEq, Debug)]
-pub struct AcceptEncoding(pub Vec<QualityItem<Encoding>>);
-
-impl_list_header!(AcceptEncoding,
-                  "Accept-Encoding",
-                  Vec<QualityItem<Encoding>>);
+header! {
+    #[doc="`Accept-Encoding` header, defined in"]
+    #[doc="[RFC7231](http://tools.ietf.org/html/rfc7231#section-5.3.4)"]
+    #[doc=""]
+    #[doc="The `Accept-Encoding` header field can be used by user agents to"]
+    #[doc="indicate what response content-codings are"]
+    #[doc="acceptable in the response.  An  `identity` token is used as a synonym"]
+    #[doc="for \"no encoding\" in order to communicate when no encoding is"]
+    #[doc="preferred."]
+    #[doc=""]
+    #[doc="# ABNF"]
+    #[doc="```plain"]
+    #[doc="Accept-Encoding  = #( codings [ weight ] )"]
+    #[doc="codings          = content-coding / \"identity\" / \"*\""]
+    #[doc="```"]
+    (AcceptEncoding, "Accept-Encoding") => (QualityItem<Encoding>)*
+}
 
 #[cfg(test)]
 mod tests {