Vote 관련 Model, Serailizer, Url, View 다시 작성
Add vote frontend (for article)
To sync vote count, use post_delete instead of pre_delete

  * pre_delete may called several times (if several request has made),
    but post_delete may called exactly once after object has really deleted
    (really ..?)
Merge pull request #17 from serialx/vote_revised

Vote 기능 개발