Add a parameter for proxy settings

[jmleroux]

We need to add a parameter to use the Icecat API through a proxy:

Without proxy :

batch.ERROR: Encountered an error executing the step: cURL error 7: Failed to connect to data.icecat.biz port 80: Connection timed out (see http://curl.haxx.se/libcurl/c/libcurl-errors.html) [] []

The additional configuration should look like this in IcecatDownloader class:

 $guzzle->request('GET', $featureListUri, [
            'auth' => $this->httpClient->getCredentials(),
            'sink' => $targetFile,
            'proxy' => 'http://login:pwd@httpproxy.mydomain.com:8080'
        ]);