Add Course.robot attribute (for automatic email generation).

Add Course.robot attribute (for automatic email generation).

I'm getting things set up to send automatic responses when mailpipe
processes incoming email, which means someone's going to have to be
signing that email for security.  Since you probably don't want to
leave your secret PGP key on the mailserver, you can now setup a new
(and less important) PGP key for each course.  The Course.robot is
just a Person instance to hold that key and an appropriate nickname.

If you want to use your own key when you call pg.py from the command
line, you can always setup a shell alias.  In Bash:

  alias pg.py='/usr/bin/pg.py --author="John Doe"'