#11 Add a required flag to the validators

[Anatol Mayen]

It was suggested on the techblog by 'Mike' that each validator also has a required flag, that tells it to either always validate or only validate when some data is put in.

Till now that is not provided by the standard validators, but can be created easily through the creation of custom validators, that reuse standard validators only if some values are put into some input field.