Commtrack consumption bug

http://manage.dimagi.com/default.asp?142400

(only real change is a813573fe440178f7f8ead72821166aabc96e1ac)
Was going to say we need to extract all of the instances of `30` into a constant, but realized `commtrack/const.py` and `consumption/const.py` both already have `DAYS_IN_MONTH = 30.`