Forms: validation data as valid JSON

See thread on forum: http://forum.nette.org/cs/14286-nevalidni-json-u-form-rules

I think it makes sense to have a valid JSON there. The used JSON.parse is [supported](http://caniuse.com/#search=json.parse) by all major browsers and by IE since version 8.
Lets discuss it here or on the forum if needed.