added a better ldap handling, and to allow query active directory

I added the ldap handling so that it could work now with e.g. activedirectory.

The pam line should look like this
auth  required       pam_yubico.so ldap_uri=ldap://ldap debug id=19 yubi_attr=pager ldapdn=dc=ad,dc=adviser,dc=com ldap_bind_user=cn=openvpn,cn=Users,dc=ad,dc=adviser,dc=com ldap_bind_password=dsjgjgfdgdger ldap_filter=(sAMAccountName=%u)