Omniauth adjustments for Shibboleth integration.

Created by: TiagoTT

Following this discussion: https://groups.google.com/forum/#!topic/gitlabhq/t860p0HWXFU

These are the changes I present to unify Omniauth provider handling and to add support to Shibboleth authentication. The omniauth-shibboleth module has been changed to improve configuration and is pending upstream approval, so I have included my own version from the Gemfile.

lib/gitlab/auth.rb

@@ -36,7 +36,7 @@ module Gitlab
           username: username,
           password: password,
           password_confirmation: password,
-          projects_limit: Gitlab.config.default_projects_limit,
+          projects_limit: Gitlab.config.gitlab.default_projects_limit,
         }, as: :admin)
         if Gitlab.config.omniauth['block_auto_created_users']