Added two buttons: Student Statistics and Assignment Statistics.  When a student is logged in and clicks on row heading “Grades:” the Student Statistics button becomes active.  If the student then clicks this button, a modal appears with a line chart displaying the students scores vs. class average on each assignment.  If there are no assignments then the modal will display the message: There is no content to display.  When a student is logged in and clicks on column heading for any assignment, the Assignment Statistics button becomes active.  If the student then clicks this button, a modal appears with a pie chart displaying the grade distribution for that assignment.

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@910012 b8457f37-d9ea-0310-8a92-e5e31aec5664