Add reporting on which credit card processor is used to bookkeeping transactions report

When a payment is received via a credit card processor, the civicrm_financial_trxn table tracks which payment processor type is being used. In 4.3 it will track the id of the payment processor. In cases of organizations with one or more payment processors, it's desirable to report on the number and amount of credit card charges that went through a specific payment processor for reconciliation purposes. It seems like the bookkeeping transactions report is one logical place to use this information both as criteria and for display.