Store credit showing up as fraction of a penny.

[Tyler Danke]

Test 1: Add 41.88 to my account using Customers->Store Credit. Run an order for 41.88. Set group pricing to 0 to avoid any interference. Observed current credit was a fraction of a penny (should have been 0). Order details do look correct (total was 0 since order was paid for by credit entirely).

We had about 600 users with negative store credit balances that are less than a $0.00001

[Jeff Lew]

We will double check and make sure all values are rounded to the currency's decimal places.

[Guilherme Padilha Maia]

Fixed rounding issues.