Add test with mock.

 * api.task()
 * api.create_task()
 * api.delete_task()
 * api.create_note()
 * api.delete_category()

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
Merge pull request #14 from mkouhei/add_some_tests

Add test with mock.