Zero dollar event fees do not get saved in early-bird discount fee table

You can save a $0 fee in Event Fees - but if you try to include the same $0 fee in a discount fee row - the row does not get saved (and no error is generated). We should be consistent and allow $0 fee rows for the discount fees.

http://forum.civicrm.org/index.php/topic,11008.msg47363.html#msg47363