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_charset.rs

@@ -1,15 +1,22 @@
 use header::{Charset, QualityItem};
 
-/// The `Accept-Charset` header
-///
-/// The `Accept-Charset` header can be used by clients to indicate what
-/// response charsets they accept.
-#[derive(Clone, PartialEq, Debug)]
-pub struct AcceptCharset(pub Vec<QualityItem<Charset>>);
-
-impl_list_header!(AcceptCharset,
-                  "Accept-Charset",
-                  Vec<QualityItem<Charset>>);
+header! {
+    #[doc="`Accept-Charset` header, defined in"]
+    #[doc="[RFC7231](http://tools.ietf.org/html/rfc7231#section-5.3.3)"]
+    #[doc=""]
+    #[doc="The `Accept-Charset` header field can be sent by a user agent to"]
+    #[doc="indicate what charsets are acceptable in textual response content."]
+    #[doc="This field allows user agents capable of understanding more"]
+    #[doc="comprehensive or special-purpose charsets to signal that capability"]
+    #[doc="to an origin server that is capable of representing information in"]
+    #[doc="those charsets."]
+    #[doc=""]
+    #[doc="# ABNF"]
+    #[doc="```plain"]
+    #[doc="Accept-Charset = 1#( ( charset / \"*\" ) [ weight ] )"]
+    #[doc="```"]
+    (AcceptCharset, "Accept-Charset") => (QualityItem<Charset>)+
+}
 
 
 #[test]