feat(headers): header ergonomics

- Reexports all common::* as header::*
- Most headers implement Deref where it makes sense.

Closes #156

src/header/common/set_cookie.rs

@@ -13,6 +13,8 @@ use cookie::CookieJar;
 #[deriving(Clone, PartialEq, Show)]
 pub struct SetCookie(pub Vec<Cookie>);
 
+deref!(SetCookie -> Vec<Cookie>)
+
 impl Header for SetCookie {
     fn header_name(_: Option<SetCookie>) -> &'static str {
         "Set-Cookie"