Works properly with OpenLDAP directories. Changes:

- Three configuration directives have been added: LDAP_OPENLDAP,
 LDAP_GROUP_MEMBER_FILTER, LDAP_GROUP_MEMBER_FILTER_FIELD
- Examples: basic_auth/app_oldap.py and groups/app_oldap.py
- Updated README.md with a more instructions

closes #16
closes #15
closes #14