Implement remaining requests check on the proxy/client

In short the ApiResponse copies the latest remainder count into the current client.
```PHP
    $instagram = new Instagram\Instagram;
    $instagram->setClientID(INSIGHTS_INSTAGRAM_CLIENT_ID);

    / /  Make lots of requests....

    $count = $instagram->getRequestsRemaining();

    if ($count < 10) {
             //  Do something about it
    }
```