Fixes #190

For the gitlab.Gitlab object make the detection of "submanagers" more
dynamic. This will avoid duplication of definitions.

Update the sphinx extension to add these managers in the list of
attributes.

Update the sphinx extension to add method definition in the docs. This
makes the documentation a bit more usable.

Hide attributes that should not have been exposed. They still exist in
the code but their documentation doesn't make much sense.