feat(headers): header ergonomics

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

Closes #156

src/header/common/accept.rs

@@ -21,6 +21,8 @@ use mime::Mime;
 #[deriving(Clone, PartialEq, Show)]
 pub struct Accept(pub Vec<Mime>);
 
+deref!(Accept -> Vec<Mime>)
+
 impl Header for Accept {
     fn header_name(_: Option<Accept>) -> &'static str {
         "Accept"