switch attendance statistics show action to index

This was originally a show action because we were displaying a single
model (CohortAttendanceStatistics). Now we want to add the ability
for a student to see their individual attendance statistics. Since
this deals with multiple attendance records, we thought it made sense to
move it to the index action to allow the show action to be for an
individual student's attendance stats.
add specs for student attendance statistics pages
add pages for student to view statistics on their attendance records
add abilities concerning student attendance statistics
change color of on time section of student attendance stats pie graph
Merge pull request #57 from epicodus/student-attendance-records

Adds page for students to view their attendance records.