Add more statuses to for the grade loader

Currently the grade loader only loads the grades distribution from ```A+``` to ```D-``` along with ```F```, and ```W```, and it cut off other 4 statuses: ```P, CR, NC, I```. However, Trends requested that we load those statuses to address some of the edge cases (i.e, pass/fail classes). 

This should be relatively small changes to ```gradeLoader.go```. 