Add another http\Client\Request to the request queue.
If the optional callback $cb returns true, the request will be automatically dequeued.
+> ***Note:***
+> The http\Client\Response object resulting from the request is always stored
+> internally to be retreived at a later time, __even__ when $cb is used.
+>
+> If you are about to send a lot of requests and do __not__ need the response
+> after executing the callback, you can use http\Client::getResponse() within
+> the callback to keep the memory usage level as low as possible.
+
See http\Client::dequeue() and http\Client::send().
+
## Params:
* http\Client\Request $request