Allow multiple confirmation messages

[jsmartt]

This PR adds the ability to set the message on a per-form basis, for the case that you have multiple forms you want checked. For example:

$('form.class1').areYouSure( {'message': 'You have unsaved changes to form 1!'} )
$('form.class2').areYouSure( {'message': 'You have unsaved changes to form 2!'} )

It's a pretty simple change, and @codedance, please review the return statement; I'm no javascript expert, so there may be some error handling that needs to be done. Also, I'm not sure that it will ever return settings.message...