refactor(lib): moved LanguageTag export to header

BREAKING CHANGE: LanguageTag used to be at the crate root, but it is now
  in the `hyper::header` module.

src/header/common/accept_language.rs

@@ -21,8 +21,7 @@ header! {
     ///
     /// # Examples
     /// ```
-    /// use hyper::LanguageTag;
-    /// use hyper::header::{Headers, AcceptLanguage, qitem};
+    /// use hyper::header::{Headers, AcceptLanguage, LanguageTag, qitem};
     ///
     /// let mut headers = Headers::new();
     /// let mut langtag: LanguageTag = Default::default();