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/user_agent.rs

@@ -0,0 +1,25 @@
+use header::Header;
+use std::fmt::{mod, Show};
+use super::from_one_raw_str;
+
+/// The `User-Agent` header field.
+///
+/// They can contain any value, so it just wraps a `String`.
+#[deriving(Clone, PartialEq, Show)]
+pub struct UserAgent(pub String);
+
+impl Header for UserAgent {
+    fn header_name(_: Option<UserAgent>) -> &'static str {
+        "user-agent"
+    }
+
+    fn parse_header(raw: &[Vec<u8>]) -> Option<UserAgent> {
+        from_one_raw_str(raw).map(|s| UserAgent(s))
+    }
+
+    fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        let UserAgent(ref value) = *self;
+        value.fmt(fmt)
+    }
+}
+