added wordpress_pingback_portscanner.rb auxiliary module

This module will connect to the XML-RPC interface that is enabled by default with the latest version of wordpress 3.5. Once connected to the XML-RPC interface it will take advantage of the Pingback API to perform portscanning on the testers behalf.

For additional details please see:
http://www.pentestgeek.com/2013/01/03/wordpress-pingback-portscanner-metasploit-module/