Checks to make sure keys used to sign exist in repository.

Added a function includesKey that checks to see if the files in people/instructor contain a given key.

When uploading grades, it first checks to see if the key used to sign the commit exists in the repository.
If it doesn't it adds the key to the class repository without overwriting existing keys.

addgrader.sh will now add keys to repository without overwriting existing keys under the same email.