Admin Privileges Based on LDAP Groups

New config option in gitlab.yml allows users in specified LDAP groups to automatically be granted admin privileges when the user is created. The config option is "admin_gids", which is an array of LDAP gids to be considered automatic admins.

**Note:** This pull request is dependent on gitlabhq/omniauth-ldap#4 in order for the LDAP strategy to obtain gid information.