Ignore zero referrals

Add a new settings "ignore_zero_referrals" under the misc settings tab, which allows you to turn off the creation of referrals for zero amounts.

Example scenario:
Customer enters a coupon code to get 100% off. The cart total is zero. 
I do not want the affiliate to get a notification to say they made zero commission.
I also do not want to store the zero commission in the referrals table.