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/date.rs

@@ -1,10 +1,17 @@
 use header::HttpDate;
 
-/// The `Date` header field.
-#[derive(Copy, PartialEq, Clone, Debug)]
-pub struct Date(pub HttpDate);
-
-impl_header!(Date, "Date", HttpDate);
+header! {
+    #[doc="`Date` header, defined in [RFC7231](http://tools.ietf.org/html/rfc7231#section-7.1.1.2)"]
+    #[doc=""]
+    #[doc="The `Date` header field represents the date and time at which the"]
+    #[doc="message was originated."]
+    #[doc=""]
+    #[doc="# ABNF"]
+    #[doc="```plain"]
+    #[doc="Date = HTTP-date"]
+    #[doc="```"]
+    (Date, "Date") => [HttpDate]
+}
 
 bench_header!(imf_fixdate, Date, { vec![b"Sun, 07 Nov 1994 08:48:37 GMT".to_vec()] });
 bench_header!(rfc_850, Date, { vec![b"Sunday, 06-Nov-94 08:49:37 GMT".to_vec()] });