refactor(header): make Quality an opaque struct

This makes the `u16` in `Quality` private, since it only has a valid
range of 0-1000, and can't be enforced in public. The `q` function now
allows both `f32`s and `u16`s to construct a `Quality`.

BREAKING CHANGE: Any use of `Quality(num)` should change to `q(num)`.

src/header/common/accept_encoding.rs

@@ -45,14 +45,14 @@ header! {
     /// );
     /// ```
     /// ```
-    /// use hyper::header::{Headers, AcceptEncoding, Encoding, QualityItem, Quality, qitem};
+    /// use hyper::header::{Headers, AcceptEncoding, Encoding, QualityItem, q, qitem};
     ///
     /// let mut headers = Headers::new();
     /// headers.set(
     ///     AcceptEncoding(vec![
     ///         qitem(Encoding::Chunked),
-    ///         QualityItem::new(Encoding::Gzip, Quality(600)),
-    ///         QualityItem::new(Encoding::EncodingExt("*".to_owned()), Quality(0)),
+    ///         QualityItem::new(Encoding::Gzip, q(600)),
+    ///         QualityItem::new(Encoding::EncodingExt("*".to_owned()), q(0)),
     ///     ])
     /// );
     /// ```