Adds Cisco SSL VPN Privilege Escalation exploit (CVE-2014-2127)

This allows you to take a level 0 SSL VPN user with clientless VPN access and priv esc to level 15 (admin) on the firewall.

Here's a video that sort of shows this in action with a little context:

https://vimeo.com/93010946

Also, I decided to use the MetaSSH plugin for this because the default SSH handler, which I believe is based on Net::SSH doesn't properly handle Cisco ASA SSH sessions interactively.  You'll get proper auth, but when you revisit the SSH session it doesn't not allow interactive usage, where MetaSSH did (I think this is due to some Linux biases that within the SSH handler that the MetaSSH plugin handles a bit more gracefully).

The SSH handling is not required to exploit or confirm the vuln, but I thought having the an actual interactive shell made it more "fun" to use.  I can always pair that back, but I'd like some feedback before I make any additional changes to this.
If you have successfully created a "backdoor" user in the device, worths to report this information and store in the database, by using the ```Msf::Auxiliary::Report``` mixin