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/lib.rs

@@ -141,6 +141,9 @@ extern crate traitobject;
 extern crate typeable;
 extern crate solicit;
 
+#[macro_use]
+extern crate language_tags;
+
 #[macro_use]
 extern crate mime as mime_crate;
 
@@ -157,6 +160,7 @@ pub use error::{Result, Error};
 pub use method::Method::{Get, Head, Post, Delete};
 pub use status::StatusCode::{Ok, BadRequest, NotFound};
 pub use server::Server;
+pub use language_tags::LanguageTag;
 
 macro_rules! todo(
     ($($arg:tt)*) => (if cfg!(not(ndebug)) {