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/connection.rs

@@ -0,0 +1,45 @@
+use header::Header;
+use std::fmt::{mod, Show};
+use super::from_one_raw_str;
+use std::from_str::FromStr;
+
+/// The `Connection` header.
+///
+/// Describes whether the socket connection should be closed or reused after
+/// this request/response is completed.
+#[deriving(Clone, PartialEq, Show)]
+pub enum Connection {
+    /// The `keep-alive` connection value.
+    KeepAlive,
+    /// The `close` connection value.
+    Close
+}
+
+impl FromStr for Connection {
+    fn from_str(s: &str) -> Option<Connection> {
+        debug!("Connection::from_str =? {}", s);
+        match s {
+            "keep-alive" => Some(KeepAlive),
+            "close" => Some(Close),
+            _ => None
+        }
+    }
+}
+
+impl Header for Connection {
+    fn header_name(_: Option<Connection>) -> &'static str {
+        "connection"
+    }
+
+    fn parse_header(raw: &[Vec<u8>]) -> Option<Connection> {
+        from_one_raw_str(raw)
+    }
+
+    fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            KeepAlive => "keep-alive",
+            Close => "close",
+        }.fmt(fmt)
+    }
+}
+