Clean up a few E_NOTICE issues.

* $_mode and $processorId are declared as static but referred to as private. This appears to be straight from [CiviCRM code sample](http://wiki.civicrm.org/confluence/display/CRMDOC/Example+of+creating+a+payment+processor+extension#Exampleofcreatingapaymentprocessorextension-3.Createthedefaultpaymentprocessorfile)
* $paymentProcessor->id is referenced but not always set (?). Check it is set before using it.
* singleton() function signature should match that of CRM_Core_Payment::singleton()