Input with type=button validated but not focused in

[denar90]

Subject of the issue

Hi guys. We've been using validator in scope of our project and discovered one interesting think.
If input type is button then it can be validated by validation rules, but handler on focusin event will not be fired. It happens because of
https://github.com/jzaefferer/jquery-validation/blob/master/src/core.js#L398-L402
handler for type=button is not set.

Environment

• version of jquery-validate - 1.15.1
• Chrome Canary - browser

Steps to reproduce

Here some fiddle - https://jsfiddle.net/yvmd3nta/

I have several ideas how to solve this.

1. Add handler for type=button
2. Add option where list of elements which will be handled can be set. Default will be current list.
3. Remove validation from inputs with type=button

I would prefer either 1 or 2 and like to make PR if you don't mind.