Implement UIResponder methods

Implement the UIResponder methods. 

- `becomeFirstResponder` will find the first invalid text field and makes it the first responder
- `resignFirstResponder` will resign the currently active field as first responder
- `canBecomeFirstResponder` will return the first invalid text field's return value for the same method
- `isFirstResponder` and `canResignFirstResponder` return the currently active fields return value for the same methods