Add AutoProxyServer

This adds automatic proxy server configuration on Windows.

When EnableAutoProxy is set (default), instances of the WebClient class will receive the appropriate proxy server for their BaseUrl.

Retrieves IE settings, including auto-detect and auto-config scripts and applies according to the specified BaseUrl.

Closes #69
Is this on GitHub somewhere? If so, include a link so that any issues and changes can be tracked there separate of VBA-Web. Otherwise, care to add one under the VBA-tools namespace? (I'll set you up with the rights if you want)