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/response.rs

@@ -1,6 +1,7 @@
 //! Client Responses
 use std::io::{self, Read};
 use std::marker::PhantomData;
+use std::net::Shutdown;
 
 use buffer::BufReader;
 use header;
@@ -42,6 +43,10 @@ impl Response {
         debug!("version={:?}, status={:?}", head.version, status);
         debug!("headers={:?}", headers);
 
+        if !http::should_keep_alive(head.version, &headers) {
+            try!(stream.get_mut().close(Shutdown::Write));
+        }
+
         let body = if headers.has::<TransferEncoding>() {
             match headers.get::<TransferEncoding>() {
                 Some(&TransferEncoding(ref codings)) => {