Split common headers into a submodule and into their own files

This is a more extensible place to put them and doesn't clutter up
header/mod.rs as much as the old scheme did.

Fixes #8

src/header/common/accept.rs

@@ -0,0 +1,44 @@
+use header::Header;
+use std::fmt::{mod, Show};
+use mime::Mime;
+
+/// The `Accept` header.
+///
+/// The `Accept` header is used to tell a server which content-types the client
+/// is capable of using. It can be a comma-separated list of `Mime`s, and the
+/// priority can be indicated with a `q` parameter.
+///
+/// Example:
+///
+/// ```
+/// # use hyper::header::Headers;
+/// # use hyper::header::common::Accept;
+/// use hyper::mime::{Mime, Text, Html, Xml};
+/// # let mut headers = Headers::new();
+/// headers.set(Accept(vec![ Mime(Text, Html, vec![]), Mime(Text, Xml, vec![]) ]));
+/// ```
+#[deriving(Clone, PartialEq, Show)]
+pub struct Accept(pub Vec<Mime>);
+
+impl Header for Accept {
+    fn header_name(_: Option<Accept>) -> &'static str {
+        "accept"
+    }
+
+    fn parse_header(_raw: &[Vec<u8>]) -> Option<Accept> {
+        unimplemented!()
+    }
+
+    fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        let Accept(ref value) = *self;
+        let last = value.len() - 1;
+        for (i, mime) in value.iter().enumerate() {
+            try!(mime.fmt(fmt));
+            if i < last {
+                try!(", ".fmt(fmt));
+            }
+        }
+        Ok(())
+    }
+}
+