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/shared/mod.rs

@@ -1,26 +1,5 @@
-//! Various functions, structs and enums useful for many headers.
-
 pub use self::encoding::Encoding;
-pub use self::encoding::Encoding::{
-    Chunked,
-    Gzip,
-    Deflate,
-    Compress,
-    Identity,
-    EncodingExt};
+pub use self::quality_item::{QualityItem, qitem};
 
-pub use self::quality_item::QualityItem;
-pub use self::quality_item::qitem;
-
-pub use self::time::tm_from_str;
-
-pub use self::util::{
-    from_one_raw_str,
-    from_comma_delimited,
-    from_one_comma_delimited,
-    fmt_comma_delimited};
-
-pub mod encoding;
-pub mod quality_item;
-pub mod time;
-pub mod util;
+mod encoding;
+mod quality_item;