Added ability for buttons to abort the dismiss of the alert view

Hi,

I've no idea if this is anything you might be interested in but I've added a parameter to the completion callback that will allow a completion block to abort dismissing the alert view. I've added an example of this to the example project as well.

I couldn't really think of an easy way to make this backwards compatible so I understand if you don't merge because of that :) (If you like the idea but want it to be backwards compatible I guess there could be a new set of constructors that take a different block format?)

Let me know what you think.

Thanks,
Sam