[Formvalidation] Support checkbox groups for "checked" rule like radio groups

[lubber-de]

Description

Checkbox groups where the checkboxes share the same name property were only "checked" validated if every single checkbox within the group was checked. This is wrong, it should behave the same as the radio group "checked" validation (SUI does it)

Hint: The example uses the same name attribute for each checkbox also with a bracket suffix "[]"
The form validator already supports using this selector by putting it into quotes .
Hint 2: selector.checkbox covers checkbox and radiobutton aswell..in case you wonder if the old behavior with radio still works

fields: {
    "sizes[]": 'checked'
  },

Testcase

Broken

Every toggle checkbox has to be checked to pass the validation
https://jsfiddle.net/ogz73ks5/

Fixed

Only one checkbox has to be checked to pass validation
https://jsfiddle.net/ogz73ks5/2/

Screenshot

Broken	Fixed

[crewow]

That's great , I Love FUI

[y0hami]

LGTM

[prudho]

LGTM