Fixed #2 - modified the AppConfigBase to default to using a null proxy if ProxyServer element not in AdWordsApi config section

Modified the AppConfigBase to default to using a null proxy (instead of WebRequest.SystemWebProxy()) if the AdWordsApi config section doesn't have a "ProxyServer" element. Using a null proxy will allow System.Net.WebRequest to find a proxy if needed and to respect the System.Net <defaultProxy enabled="false" /> app config setting