Primary changes:
1. Added ExceptionHandler.
2. Added JQuery-form-plugin to provide the functionality of submitting
forms in ajax style.
3. Added images previewing/adding functionalities w/o combining with
server-side.
4. Added AuthorizationController to calculate the signatures and options
for the requests sent to the UpYun storage.
5. Enhanced the ability of ArticleController.
6. Updated the related test-cases.
7. Updated the sample page: CreateArticlePage.jsp