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
I noticed ```Option Explicit``` was not set in WebHelpers, is this voluntary ?
On a different note, I like using ```Option Private Module``` because this way, public functions are not accessible from Excel formulas, introducing less confusion for end-users.