Statistics not updated after deleting learner's state

[kitsook]

With issue #119 fixed, instructors will be able to delete learner's state. However, statistics on original/revised answer wont be updated.

Whenever an answer is submitted, statistics are updated to count how many time each answer is selected. And the statistics are used to draw the charts after students submitted revisions.

Technical details
The statistics are stored as XBlock field with Scope.user_state_summary scope. This information is not available to the XBlock when edx platform invokes the callback clear_student_state when deleting learner's state. And thus XBlock cannot update the stats.