new option error_target

This is useful to ensure only 1 error is shown per control group, and lets you control its location (say below the checkboxes).

Example usage:

    $('#form').isHappy({
          fields: {
           'input[name=style]': {
              required: true,
              message: 'What style of photos do you prefer?',
              error_target: '#style',
              when: 'change',
              test: function(){ 
                return $('input[name=style]').is(':checked');
              },
            },
        });