Added post module to get a MITM through a pptp tunnel

The module creates an outboud VPN connection (pptp) to a VPN server using the rasdial Windows client. We can choice to forward all victim traffic through the tunnel getting a man-in-the-middle attack.

I have done the tests using  pptpd as a VPN server in a Debian 6 machine. 
Added a how-to video in the references.