Added the grade command and functionality so that users can assign grades to their assignments and see the percentage result. This also adds to the show command that the user can see their final grade in the course that is loaded. Changed the assignments table a bit, now the value for percent worth in the class of an assignment is stored as a double, so that there's more granularity in assigning weights to assignments. Now assignments can be worth things like 1.2% of the final course grade, which happens... all the time, so I thought it was a good idea to have that possiblilty.