DotNet: support system-level proxy settings

The DotNet proxy script did not obey system-level proxy settings (to be specific, the Internet Explorer proxy settings of the currently impersonated user). Because of this, the script will fail in environments which require a web proxy for outgoing connections. This change adds support for system proxy settings on outgoing http web requests through [WebRequest.GetSystemWebProxy](http://msdn.microsoft.com/en-us/library/system.net.webrequest.getsystemwebproxy(v=vs.110).aspx). The change is backward compatible in the sense that, if no system proxy is configured for the current user, requests will still default to a direct connection.

Tested on Windows 7 Enterprise and Windows Server 2012 R2, IIS 8.5, .NET 4.5.2, on REST queries sent to ArcGIS Server, Geoserver, and OSM Overpass API.