feat(langtags): use true language tags in headers

Make hyper dependant on rust-language-tags providing complete parsing
and formatting of language tags. Remove builtin solution for simple
tags.

BREAKING CHANGE: AcceptLanguage and ContentLanguage use LanguageTag now,
Language removed from Hyper.

src/header/shared/mod.rs

@@ -2,12 +2,10 @@ pub use self::charset::Charset;
 pub use self::encoding::Encoding;
 pub use self::entity::EntityTag;
 pub use self::httpdate::HttpDate;
-pub use self::language::Language;
 pub use self::quality_item::{Quality, QualityItem, qitem, q};
 
 mod charset;
 mod encoding;
 mod entity;
 mod httpdate;
-mod language;
 mod quality_item;