Reverse http proxy pstore

This should be an update to the closed pull request "new stager reverse_http_proxy_pstore" from January 28th 2013: https://github.com/rapid7/metasploit-framework/pull/1396
But I'm not able to reopen that pull request.

In response to wchen-r7's feedback in the old pull request, I corrected the way the address of functions from pstorec.dll are obtained. Testing was on Windows Server 2003 and XP.