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 used? If you want, I would feel comfortable using `Err.Raise` in here and then handling the error on the `WebClient` side (probably just log it and ignore it).