Add ability to adjust customer balance

[psincraian]

trafficstars

Prorations are a complex feature, if we add refunds and take into account the customer balance there is no behaviour that fit all the merchants.

Case 1

1. Customer buys annual subscription of $100/year
2. Customers moves to monthly sub $10/month (with a balance of $90)
3. Customer requests a refund of $90 of the first order (balance now is $0)

Case 2

1. Customer buys annual subscription of $100/year
2. Customers moves to monthly sub $10/month (with a balance of $90)
3. Customer requests a refund of $100 of the first order (balance now is -$10)