Do not add properties 'errors', 'isValid' and 'isAnyMessageShown' to grouped obj

The group function surprised me a bit by adding the following properties to view model:

``` javascript
var vm = {};
var group = ko.validation.group(vm);
typeof vm.errors === 'function'; // true
typeof vm.isValid=== 'function'; // true
typeof vm.isAnyMessageShown=== 'function'; // true
```

I stumbled over that after trying to find out why some unit tests failed (the reason being these properties).

Therefore I have added a new option ```addResultToVM``` to group which configures if to set these properties.

**I have set that configuration to ```false``` because I think we should follow the principle of least surprise. But that might break existing code.**