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/language.rs

@@ -1,45 +0,0 @@
-use std::str::FromStr;
-use std::fmt;
-
-/// A language tag.
-/// See http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.10
-///
-/// Note: This is no complete language tag implementation, it should be replaced with
-/// github.com/pyfisch/rust-language-tag once it is ready.
-#[derive(Clone, PartialEq, Debug)]
-pub struct Language {
-    /// The language tag
-    pub primary: String,
-    /// A language subtag or country code
-    pub sub: Option<String>
-}
-
-impl FromStr for Language {
-    type Err = ::Error;
-    fn from_str(s: &str) -> ::Result<Language> {
-        let mut i = s.split("-");
-        let p = i.next();
-        let s = i.next();
-        match (p, s) {
-            (Some(p), Some(s)) => Ok(Language {
-                primary: p.to_owned(),
-                sub: Some(s.to_owned())
-                }),
-            (Some(p), _) => Ok(Language {
-                primary: p.to_owned(),
-                sub: None
-                }),
-            _ => Err(::Error::Header)
-        }
-    }
-}
-
-impl fmt::Display for Language {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        try!(f.write_str(&self.primary[..]));
-        match self.sub {
-            Some(ref s) => write!(f, "-{}", s),
-            None => Ok(())
-        }
-    }
-}

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;