Rewrite tests: add test_initial(), test_command()
Use send-notify to show the notification
Update doc
Rewrite tests: use unittest
Fix chinese character in story_content

or it will cause this error:
    File "wonderful_bing/wonderful_bing.py", line 46, in show_notify
        notify_icon, title, story_content))
    codeEncodeError: 'ascii' codec can't encode characters in position 0-8:
    ordinal not in range(128)
Refine test

- add tearDown
- add test_picture_has_been_downloaded
Refine test

- add test_without_directory
- add test_directory_not_exist
Add check for the not existing path
Add check if the path not end with '/'
Pep8 check for test.py