Store violation list in validationResult

There is no easy way to get the list of violations after a call to `parsleyForm.validate()`, but I think it would be very useful.

This PR stores the list of violations in `form.validationResult`.
This is potentially a breaking change, though, if anyone depends on `form.validationResult` being `false`. As it isn't documented, I thought it's ok, but maybe we better add another attribute instead (`violations`?).