v1.3

• Fixed an issue where multiple checkboxes could not be validated.
• There has been an API-breaking change made to callback behavior:

1. If the required rule is present, a callback will be fired once all other validation rules pass.
2. If the field is not required and it is empty, the callback will not be called unless condition #3 is met.
3. A callback will always be called if it is preceded by an '!' i.e. rules: '!callback_myCustomCallback'