refactor(headers): export all headers and utils directly under header

Currently headers are exported at many places. For example you can access
`Transfer-Encoding` header at `header`, `header::common` and
`header::common::transfer_encoding`. Per discussion on IRC with
@seanmonstar and @reem, all contents of headers will be exposed at `header`
directly. Parsing utilities will be exposed at `header::parsing`. Header
macros can now be used from other crates.

This breaks much code using headers. It should use everything it needs
directly from `header::`, encodings are exposed at `header::Encoding::`,
connection options are exposed at `header::ConnectionOption`.

src/header/common/accept_encoding.rs

@@ -1,22 +1,22 @@
-use header::{self, shared};
+use header::{self, 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, Show)]
-pub struct AcceptEncoding(pub Vec<shared::QualityItem<shared::Encoding>>);
+pub struct AcceptEncoding(pub Vec<QualityItem<Encoding>>);
 
 impl_list_header!(AcceptEncoding,
                   "Accept-Encoding",
-                  Vec<shared::QualityItem<shared::Encoding>>);
+                  Vec<QualityItem<Encoding>>);
 
 #[test]
 fn test_parse_header() {
     let a: AcceptEncoding = header::Header::parse_header([b"gzip;q=1.0, identity; q=0.5".to_vec()].as_slice()).unwrap();
     let b = AcceptEncoding(vec![
-        shared::QualityItem{item: shared::Gzip, quality: 1f32},
-        shared::QualityItem{item: shared::Identity, quality: 0.5f32},
+        QualityItem{item: Encoding::Gzip, quality: 1f32},
+        QualityItem{item: Encoding::Identity, quality: 0.5f32},
     ]);
     assert_eq!(a, b);
 }