Change how invoice email is done to an observer event

Changed how the order invoice emails are sent.

The current way using Fontis_EwayAu_Model_Direct::processInvoice is flawed.

Customers mistake the invoice email for the success email, as it is received first.

If something happens during the order process, (after this method is called) the client will receive an email with the invoice, which states an invoice id for them.
If (for some reason) the reserved invoice id is rolled back, you will find two clients with emails that have the same invoice id. (This has occurred on a site recently, which prompted this change)

The sending has been moved to an event that runs after the order objects have been successfully created and saved.
This then also negates the need to force save the invoice object (as per old code)