Changed fetch_pages to fetch_all_pages and its behavior, etc.

- fetch_all_pages fetches all wordbook pages without url list as its
  argument. It should be able to recognize the end of wordbook page..
  but the algorithm is not perfect because it was found that small
  changes exist between pages which index are last and last + 1.
- fetch_page fetches page contents as UTF-8.
- parse_and_save receives scalar, not array.
- According to these changes, some tests are also changed.