Implement DateField to access arbitrary dates
Introduce ACLField

This is a special field type. There are LDAP servers that use a special ACL
attribute to store the user's groups. Example:
ACL = ['user.group', 'trustee.group']

This allows us to do the following in the ldapdb model:
is_user = ACLField(db_column='ACL')
is_developer = ACLField(db_column='ACL')
is_trustee = ACLField(db_column='ACL')

For the above example, it will return binary results for each one of the above
properties:
ldapuser.is_user = True
ldapuser.is_developer = False
ldapuser.is_trustee = True
Support negative lookups for ACLField

Example:
non_developers = LDAPUser.objects.filter(is_developer=False)