Added cron file

This is the "easy" method to use a dedicated file for cron. If you accept this, you will have to update both the docs in brewpi-www to reflect an updated cron line, and (you or I) can update the install.sh to add the new cron line (it will be brewpi.cron, instead of brewpi.py). Note, leaving the "old" cron line in won't hurt users who don't know to update it. 

I am going to submit another pull request that is a little more in-depth, that can first check if brewpi.py is running, and only then re-run it. You can accept either one, but I'd recommend comparing them both to see which one has more utility in the long run. For what its worth, the one I'm about to work on next is probably considered more of a "good" solution in the larger coding community, but I think either one would meet your needs