Authenticate LDAP users in the grack module

Created by: jasl8r

This resolves issue #2012 (closed).

I had suggested some options in the original pull request for this issue, but in the end I used the omniauth-ldap module directly to perform the authentication. This does two things, it keeps the code in the grack_auth module simpler, and it ensures that any LDAP repository and settings working with devise and the main site should work here.