Validation errors and offline form type

I've made two changes:
1. Make sure that a failed validation always triggers the ```onError``` handler. The only chance to act on a failed validation was if the method (insert, update) failed because of an invalid doc. If you wanted to use your own ``data-meteor-method`` there wouldn't be any error triggered and your method would just be skipped silently.
2. I added a form type called ```offline```. I've got a use case where I don't want to trigger any server action at all - no insert, update or custom meteor method and no normal form submit. Just a client-side ```onSubmit``` handler. This is now possible by adding ```class="offline"``` to your submit button.