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`.

examples/server.rs

@@ -6,7 +6,7 @@ use std::io::util::copy;
 use std::io::net::ip::Ipv4Addr;
 
 use hyper::{Get, Post};
-use hyper::header::common::ContentLength;
+use hyper::header::ContentLength;
 use hyper::server::{Server, Request, Response};
 use hyper::uri::RequestUri::AbsolutePath;