Add ability to validate that at least one of several checkboxes is checked

Example:

```html
Select at least one fruit:
<input type="checkbox" name="fruit-apple" data-oneof="fruit"> Apple <br />
<input type="checkbox" name="fruit-banana" data-oneof="fruit"> Banana <br />
<input type="checkbox" name="fruit-mango" data-oneof="fruit"> Mango
```