fix these methods in the new model

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Strict Standards: Declaration of Invoice::save() should be compatible with that of Doctrine_Record::save() in /Users/enrique/Documents/workspace/siwapp/lib/model/doctrine/Invoice.class.php on line 6

Strict Standards: Declaration of InvoiceItem::delete() should be compatible with that of Doctrine_Record::delete() in /Users/enrique/Documents/workspace/siwapp/lib/model/doctrine/InvoiceItem.class.php on line 112

Strict Standards: Declaration of InvoiceItemTax::delete() should be compatible with that of Doctrine_Record::delete() in /Users/enrique/Documents/workspace/siwapp/lib/model/doctrine/InvoiceItemTax.class.php on line 39

Strict Standards: Declaration of Payment::delete() should be compatible with that of Doctrine_Record::delete() in /Users/enrique/Documents/workspace/siwapp/lib/model/doctrine/Payment.class.php on line 39

Strict Standards: Declaration of RecurringInvoice::save() should be compatible with that of Doctrine_Record::save() in /Users/enrique/Documents/workspace/siwapp/lib/model/doctrine/RecurringInvoice.class.php on line 280

Strict Standards: Declaration of RecurringInvoiceItem::delete() should be compatible with that of Doctrine_Record::delete() in /Users/enrique/Documents/workspace/siwapp/lib/model/doctrine/RecurringInvoiceItem.class.php on line 109

Strict Standards: Declaration of RecurringInvoiceItemTax::delete() should be compatible with that of Doctrine_Record::delete() in /Users/enrique/Documents/workspace/siwapp/lib/model/doctrine/RecurringInvoiceItemTax.class.php on line 36

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r857