Account Type Event Scripts; added new event scripts for calculating Last Payment, Last Reversed Payment and Next Payment

New event scripts have been added for calculating the Last Payment, Last Reversed Payment and Next Payment for an Account.

These scripts can be defined for each Account Type under the Scripts page and allow specialised logic to be implemented.

The Script must be defined as an "Object Events" type with the object set as "Account". Use the "LastPaymentCalculate", "LastReversedPaymentCalculate" and "NextPaymentCalculate" events.