Make submit delegated handler behave like knockout

References #5 (https://github.com/rniemeyer/knockout-delegatedEvents/issues/5).

Knockout's submit binding handler is subtly different to the other event handlers.

Instead of the root element being `this`, and getting two arguments `data`, and `event`, it sets `this` to the current bindingContext value, and passes a single argument that is the form element that is bound.

This patch replicates that behaviour.