Allow validator chaining, added URL validator

* Validators should now be called via JCValidators::run()
* JCObjContent::check() now accepts arrays of validators
* Added JCValidators::getUrlValidator()
* Added JCMissing class to specify that a certain value was not given
* Added JCObjContent::required()

Change-Id: I15454fc0597a67605b84a61c5ff3b17da7acb082
Merge "Allow validator chaining, added URL validator"