Add tax calculations to return authorization refunds

When "ReturnAuthorization#process_refund" is called, tax is calculated on each return item and stored in the database and then the refund is performed.
Taxation is broken out like this to allow easy third-party tax api integration.

Also: Moved some rounding further out since it was causing small but noticeable differences in refund amounts (off by 8 cents sometimes, for example).