Update the client API to statically track headers and move constructors

The client Request now uses the same system as a server Response to track
the write status of headers, and the API has been updated accordingly.

Additionally, the Request constructors have been moved onto the Request object
instead of being top-level hyper functions, as this better namespaces the
client and Server.

src/lib.rs

@@ -15,7 +15,6 @@ extern crate intertwine;
 pub use std::io::net::ip::{SocketAddr, IpAddr, Ipv4Addr, Ipv6Addr, Port};
 pub use mimewrapper::mime;
 pub use url::Url;
-pub use client::{get, head, post, delete, request};
 pub use method::{Get, Head, Post, Delete};
 pub use status::{Ok, BadRequest, NotFound};
 pub use server::Server;