Critical Index fix + changed init

Refactored Init method and removed adding the group to the touch responder.

Also, critical bug: indiscriminately incrementing currentResponderIndex when there are no responders left in the chain cause the next touch to be ignored. See these lines [191-195](https://github.com/blender/MGTouchResponder/blob/master/MGTouchResponder/Classes/MGTouchResponderGroup.m#L191-195)