Add group api, removed user_teams api

Dependant on https://github.com/gitlabhq/gitlabhq/pull/4990
prettify error message
fix compatibility with API v3; update docs
Merge branch 'feature-group-api' of github.com:karlhungus/gitlab into karlhungus-feature-group-api

Conflicts:
	spec/gitlab/client/user_teams_spec.rb
fix docs