Change about page into a popup dialog of index page
Change delete page into a popup dialog of form page
Fix bug: clear form when displayTask is called with undefined task
Implement add new task/modify old task
Implement remove a task
Handle datebox
Remove form.html and define a new div as formPage in index.html --> single page app
Fix problem in the last commit: actually remove form.html
Remove script tags to the bottom of index.html