small bug fixes

Thanks for this library, super helpful! Here are a couple fixes for small things I ran into:

(1) There was a confusion where the check for using separate expMonth, expYear was actually checking for expiryMonth, expiryYear which made it impossible to use them. If you used the long named, _getDataToSend wouldn't fill them in; if you used the short names it would try to use scope.expiry.

(2) Angular makes it really hard to be sure a form that isn't ng-valid doesn't get submitted, so if it does, I simulate a 400 status call to the user's stripe handler. That gives a good place to put up an error message and turn off any spinners.

Hope you find these useful!