include CustomObjectKeyConstructor CustomListKeyConstructor and change_api_updated_at in the library

[inspirassion]

def change_api_updated_at(sender=None, instance=None, *args, **kwargs):
    cache.set('api_updated_at_timestamp', datetime.datetime.utcnow())

class UpdatedAtKeyBit(KeyBitBase):
    def get_data(self, **kwargs):
        key = 'api_updated_at_timestamp'
        value = cache.get(key, None)
        if not value:
            value = datetime.datetime.utcnow()
            cache.set(key, value=value)
        return force_text(value)

class CustomObjectKeyConstructor(DefaultKeyConstructor):
    retrieve_sql = RetrieveSqlQueryKeyBit()
    updated_at = UpdatedAtKeyBit()

class CustomListKeyConstructor(DefaultKeyConstructor):
    list_sql = ListSqlQueryKeyBit()
    pagination = PaginationKeyBit()
    updated_at = UpdatedAtKeyBit()

This logic must be directly included in the framework not just as exemple in documentation, It's perfect and ready to be uses.