feat(headers): header ergonomics

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

Closes #156

src/header/common/user_agent.rs

@@ -8,6 +8,8 @@ use super::util::from_one_raw_str;
 #[deriving(Clone, PartialEq, Show)]
 pub struct UserAgent(pub String);
 
+deref!(UserAgent -> String)
+
 impl Header for UserAgent {
     fn header_name(_: Option<UserAgent>) -> &'static str {
         "User-Agent"