add generic 'callback' validator

I added a generic callback validator.

The following example will only validate, if the entered value equals "foobar".
Of course, you can do a lot more complex stuff those callback functions.

```javascript
Regulate('demoForm', [
  { name: 'someField',
    callback: function(value){
        if (value == 'foobar'){
          // this will pass
          return true;
        }else{
          // any none true value will be displayed as error message
          return 'The value must be "foobar", not "' + value + '"';
        }
      },
    display_error: '#someFieldError'
  },
]);