Tidy up cancelRecurring code

We have an old syntax in use for the cancel recurring. Ideally we want to change FROM

$paymentObject->isSupported()

to

$paymentObject->supports()

and remove the isSupported method entirely.

We also want to change

$paymentObject->cancelRecurring(string, array)

{ return something odd; }

to

$paymentObject->doCancelRecurring(array)

{ return array or throw exception; }

Once again for consistency with other methods

I'll note the similar desired changes for editRecurring shortly - but these also include allowing the processor to specify editable fields - e.g IATS, eway would permit editing next_sched_contribution_date

$paymentObject