Update mail.py

Add JOBS to subject line for digest. Requested per https://listserv.nd.edu/cgi-bin/wa?A2=ind1405&L=CODE4LIB&F=&S=&P=205534 The listserv now has JOBS as a topic, per http://blogs.nd.edu/emorgan/2014/05/jobs-topic/ and ELM code4lib post 2014-05-15.