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.
In the case that a command stager is used ```execute_command``` may be executed multiple times. That would lead to this module logging in more than once. If it's not necessary to login each time to execute a command, perhaps you could get the session_cookie in the ```exploit``` function to save some requests.