Name AFHTTPClient signals

As described in RAC's Best Practices, [Debug streams by giving them names](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/f9bc1fd2c58e0c5f9096a1c42d3e51d9c912d4e0/Documentation/DesignGuidelines.md#debug-streams-by-giving-them-names).