feat(rxForm): Fixes required attribute to checkboxes in rxOptionTableForm

Resolves #377 .
After reading through this code, the functionality intended by this array is to simply determine if there are any checkboxes checked. I think that, instead of going through the complexity of storing this in an array, we could simple have a counter for the number of checked checkboxes:
```
var boxesChecked = 0;
$scope.updateCheckboxCount = function (val, index) {
    if ((val === true) || $scope.data[index].value == val) {
        boxesChecked += 1;
    } else {
        boxesChecked -= 1;
    }
};

$scope.checkIfRequired = function () {
    if (_.isBoolean($scope.required)) {
        return $scope.required && boxesChecked === 0;   
    } else {
        return false;
    }
};
```