Made currently validating field accessible to callbacks.

Adds a 'validating' variable used to make the current field accessible to callbacks. Necessary to implement a 'don't validate if not required clause' in the callback.

That said, maybe it's worth implementing a switch in the rules string (I think an exclamation mark would be ideal), that makes running the callback conditional on an empty field.

For example
['required|callback_foo'] = callback foo will always be called
['!callback_foo'] = callback foo will always be called
['other_rules|callback_foo'] = callback foo will only be called if the field is not empty

**edit: Actually probably better the other way around to prevent breaking backwards compatibility of api, so:**
['required|callback_foo'] = callback foo will always be called
['callback_foo'] = callback foo will always be called
['other_rules|!callback_foo'] = callback foo will only be called if the field is not empty

Happy to implement.