Allow EconomytransactionEvent to be cancelable

[turtledude01]

It would be nice to be able to cancel the transaction for various reasons, the main one being the idea behind my plugin ServerEco. It listens for the transaction and would cancel it if another account cant handle it.

[turtledude01]

I'm not sure myself on this, but if canceling the event were to cause the ResultType to be ResultType.canceled I believe that would allow plugins to do handling depending on what the economy plugin/listener does specifically. I believe more flexibility would be good. And this would also not cause most well made plugins currently to break, since most either just listen for the ResultType.SUCCESS or listen for all other result types.