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/allow.rs

@@ -1,14 +1,19 @@
 use method::Method;
 
-/// The `Allow` header.
-/// See also https://tools.ietf.org/html/rfc7231#section-7.4.1
-
-#[derive(Clone, PartialEq, Debug)]
-pub struct Allow(pub Vec<Method>);
-
-impl_list_header!(Allow,
-                  "Allow",
-                  Vec<Method>);
+header! {
+    #[doc="`Allow` header, defined in [RFC7231](http://tools.ietf.org/html/rfc7231#section-7.4.1)"]
+    #[doc=""]
+    #[doc="The `Allow` header field lists the set of methods advertised as"]
+    #[doc="supported by the target resource.  The purpose of this field is"]
+    #[doc="strictly to inform the recipient of valid request methods associated"]
+    #[doc="with the resource."]
+    #[doc=""]
+    #[doc="# ABNF"]
+    #[doc="```plain"]
+    #[doc="Allow = #method"]
+    #[doc="```"]
+    (Allow, "Allow") => (Method)*
+}
 
 #[cfg(test)]
 mod tests {