ldap filters

This pull request will enable filter queries against ldap which is particularly useful when you have a big ldap directory and want to give access only to a subset of your users to your gitlab instance.

This pull request will enable this feature/functionality after this pull (https://github.com/gitlabhq/omniauth-ldap/pull/3) request is merged in.

Details of mentioned pull request:
Please note that this pull request was migrated from omniauth-ldap project pull request 22 which was developed and authored by sdeframond and can be accessed at this link: intridea#22