Remove token-requirement on APIs that work without authentication

[dotWee]

Some of the APIs GitLab provides are usable without a user generated token, like the /users/:user_id/projects path (reference):

Get a list of visible projects owned by the given user. When accessed without authentication, only public projects are returned.

It would be awesome to use those without getting an token is mandatory error!

[jdalrymple]

For the time being, I could remove the requirement for a token and anything that does require a token would just error out :s not the most elegant solution but until we add the proposed mapping in #104 , it would be a waste of time to implement throughout all the services.

[dotWee]

Please correct me if I'm wrong, but wouldn't error'ing out result in just like any request with an invalid token? aka. the implemented StatusCodeError: 401 Unauthorized
Anyway, an implementation after #104 seems reasonable.

[jdalrymple]

Yup! Which is still pretty clear of a message. Ill make a PR this weekend.

[michael-smt]

It would be great to have this possibility again in the newest version.