1107 from sunrise
[modify exercise]
Add hints to skip_counting_by_10s.html.
Unbind keyup keypress events after validating answer

- Keyup event may leads to the check-answer button of the next question
  being disabled.
[add <code>]
Add <code> for all numbers.
Modify content

- Change to CH_TW
- Change variable
- Modify graph by for loop
Merge branch 'skip_counting_by_10s'
revise wording by ej
Merge branch 'multiplication_and_division_word_problems'