Getting start with IXMLHTTPRequest

[Nikolay Sivov]

Hi.

Recently I tried to start with this interface. It looks trivial enough 
but since it's first time i'm using winhttp,
there could be some problems, especially cause IXMLHTTPRequest supports 
asynchronous requests.

Could someone with winhttp knowledge review this patch before it goes to 
far from reality?

P.S. any comments are welcome actually.