Adds get_groups method to provide an interface for

listing all available groups in the ldap directory
based on the filter defined in LDAP_GROUPS_OBJECT_FILTER

The get_groups method has two arguments fields and dn_only
to customize the result. i.e.
get_groups(fields=['cn', 'description'])
provide a list of dict with included attributes. if only
get_groups(dn_only=True) is provided, a list of available
groups dn is returned. If no users in directory, an empty
list is returned.