Added a student statistics button. When a student is selected and the button is clicked a student statistics modal appears displaying student scores vs. class average for all assignments to date.  If there are no assignments, then the modal contains the message: There is no content to display.

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@909929 b8457f37-d9ea-0310-8a92-e5e31aec5664