feat(client): add a Connection Pool

This adds a connection pool to the Client that is used by default. It
accepts any other NetworkConnector, and simply acts as a
NetworkConnector itself. Other Pools can exist by simply providing a
custom NetworkConnector. This Pool is only used by default if you also
use the default connector, which is `HttpConnector`. If you wish to use
the Pool with a custom connector, you'll need to create the Pool with
your custom connector, and then pass that pool to the
Client::with_connector.

This also adds a method to `NetworkStream`, `close`, which can be used
to know when the Stream should be put down, because a server requested
that the connection close instead of be kept alive.

Closes #363
Closes #41

src/http.rs

@@ -7,7 +7,8 @@ use std::fmt;
 use httparse;
 
 use buffer::BufReader;
-use header::Headers;
+use header::{Headers, Connection};
+use header::ConnectionOption::{Close, KeepAlive};
 use method::Method;
 use status::StatusCode;
 use uri::RequestUri;
@@ -443,6 +444,15 @@ pub const LINE_ENDING: &'static str = "\r\n";
 #[derive(Clone, PartialEq, Debug)]
 pub struct RawStatus(pub u16, pub Cow<'static, str>);
 
+/// Checks if a connection should be kept alive.
+pub fn should_keep_alive(version: HttpVersion, headers: &Headers) -> bool {
+    match (version, headers.get::<Connection>()) {
+        (Http10, Some(conn)) if !conn.contains(&KeepAlive) => false,
+        (Http11, Some(conn)) if conn.contains(&Close)  => false,
+        _ => true
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::io::{self, Write};