feat(headers): adds Accept

Moved utils to shared/. Added quality_value.

src/header/common/user_agent.rs

@@ -1,6 +1,6 @@
 use header::{Header, HeaderFormat};
 use std::fmt::{mod, Show};
-use super::util::from_one_raw_str;
+use header::shared::util::from_one_raw_str;
 
 /// The `User-Agent` header field.
 ///