feat(headers): header ergonomics

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

Closes #156

src/header/common/date.rs

@@ -9,6 +9,8 @@ use time::{Tm, strptime};
 #[deriving(PartialEq, Clone)]
 pub struct Date(pub Tm);
 
+deref!(Date -> Tm)
+
 impl Header for Date {
     fn header_name(_: Option<Date>) -> &'static str {
         "Date"