Initial test for partial payments
Merge branch 'master' of github.com:killbill/killbill
Introduce new query parameter (paymentAmount) when paying all invoices on an account
If the amount is less than the account balance then the earliest unpaid invoices may not be paid (or may partially be paid)
If the amount is greater than the account balance and if this is for an external payment (check), we apply a credit on the account.