edited calcgrade.sh and install-instructor-keys.sh

Install instructor keys should now export they key most recently generated from within the script. If multiple keys exist under the same email, it should no longer export all of them.

calcgrade will no longer say [bad signature] when there's an unverified signature. Students who run the script will almost never verified instructor signatures. This still allows instructors to notice anyone masquerading as an instructor.

Fixed a minor typo in config.sh