Sync with intridea/omniauth-ldap

There are some useful updates in https://github.com/intridea/omniauth-ldap. This merges the current master of https://github.com/intridea/omniauth-ldap into https://github.com/gitlabhq/omniauth-ldap. This merge should handle PR #3, PR #6 and PR #7.

Why this as opposed to PR #9? It appears PR #9 missed some commits from https://github.com/intridea/omniauth-ldap and those meet the needs of PR #3.