Fix Valid Toggle

Fix `validToggler()` to correctly toggle on Expiry Date. Seems like
`jquery.payment` couldn't work with formatted date, as parsed by `val`.
I might be stupid, but this is the only way I got it to run.
Where would you recommend to bind the data attribute to? I would also like to bind the expiry date object to something.(to fix issue #58) The object is then available and the user can then do whatever they want. Does `validToggler()` need to be external to `card` anyway?