[4.2] add setter and getter for Validatior

for #4037 and #4714
There is already quite similar method called [setAttributeNames](https://github.com/laravel/framework/blob/4.2/src/Illuminate/Validation/Validator.php#L2116).

In this case, perhaps the other two method should be called `setValueNames` and `getValueNames`?

the getCustomAttributes should be `getAttributeNames`?
I know your naming makes more sense, but `setAttributeNames` is already in API.