xmlclient doesn't support HTTP/HTTPS Proxy

[ellieayla]

https://github.com/idbentley/pyforce/blob/master/src/pyforce/xmlclient.py#L40 should probably be enhanced to support connecting through a HTTP Proxy similar to superfell/Beatbox#5 commit superfell/Beatbox@76aff60 or superfell/Beatbox@9533fa3