Add gitlab-shell command injection module

This request adds a module for gitlab-shell command
injection for versions prior to 1.7.4. This has been
tested by installing Gitlab version 7.1.1 on Ubuntu and then
using information at http://intelligentexploit.com/view-details.html?id=17746
to modify the version of gitlab-shell to a vulnerable one. This
was done as I could not find a better method for downloading
and deploying an older, vulnerable version of Gitlab.
I don't think you need to scan the cookie and flatten it. Just use `res.get_cookies` and send all the received cookies with the following requests. This way you reduce complexity of the module and don't ignore other cookies that may be set (like a session stickyness loadbalancer cookie). The same applies for then `@session_cookie` variable below.
Or do you need to send exactly one cookie to the server?