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/last_modified.rs

@@ -2,8 +2,8 @@ use std::fmt;
 use std::str::FromStr;
 use time::Tm;
 use header::{Header, HeaderFormat};
-use header::shared::util::from_one_raw_str;
-use header::shared::time::tm_from_str;
+use header::parsing::from_one_raw_str;
+use header::parsing::tm_from_str;
 
 /// The `LastModified` header field.
 #[derive(Copy, PartialEq, Clone, Show)]