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/client/request.rs

@@ -1,6 +1,7 @@
 //! Client Requests
 use std::marker::PhantomData;
 use std::io::{self, Write, BufWriter};
+use std::net::Shutdown;
 
 use url::Url;
 
@@ -8,7 +9,7 @@ use method::{self, Method};
 use header::Headers;
 use header::{self, Host};
 use net::{NetworkStream, NetworkConnector, HttpConnector, Fresh, Streaming};
-use http::{HttpWriter, LINE_ENDING};
+use http::{self, HttpWriter, LINE_ENDING};
 use http::HttpWriter::{ThroughWriter, ChunkedWriter, SizedWriter, EmptyWriter};
 use version;
 use HttpResult;
@@ -154,7 +155,10 @@ impl Request<Streaming> {
     ///
     /// Consumes the Request.
     pub fn send(self) -> HttpResult<Response> {
-        let raw = try!(self.body.end()).into_inner().unwrap(); // end() already flushes
+        let mut raw = try!(self.body.end()).into_inner().unwrap(); // end() already flushes
+        if !http::should_keep_alive(self.version, &self.headers) {
+            try!(raw.close(Shutdown::Write));
+        }
         Response::new(raw)
     }
 }