Implement gitlab 2FA support

[guyzmo]

The same way github's 2FA support has been recently added (cf #39) implement gitlab's 2FA as well.

Even though implementing 2FA with github was easy thanks to @sigmavirus24's work on sigmavirus24/github3.py where it was already implemented… Doing it on gitlab needs to start with patching gpocentex/python-gitlab to implement a 2FA callback… Looking at the code it looks like there's no mechanism in place for that yet:

• Gitlab.credentials_auth()

That being said, it is unclear how to use 2FA and whether a mechanism is in place for using it with Gitlab's API, as there's no mention of 2FA in the authentication section of the Gitlab API documentation.

[guyzmo]

ok after skimming through the gitlab API documentation, if you enable 2FA, you cannot login using login+password on the API, and when you use a token (either personal access token or privatekey) you don't have 2FA.

So, as long as this is not supported by gitlab itself, I'll mark this issue as wontfix.