add is_object() method

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
change api of is_container(), response is status code to boolean

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
increase version and update documents

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>