Added support for a port number in the AFHTTPClient baseURL.

The current loadRemoteImageForURL method does not define a port number and therefore every image is requested over port 80. This commit adds support for a port number.

By the way, NSURL docs state that [imageURL port] should return nil in case the port does not conform to RFC 1808. I tested URLs with and without port numbers but both seem to work correctly. I suppose the AFHTTPClient handles both cases.