code: function validateField ( fldID, fieldMeta, value ) {
          var res  = {}
            , errs = checkByAll( fieldMeta.validators, value, fieldMeta );
          res[ fldID ] = errs.length ? errs : null;
          return res;
      }
docs: Calculates field's validity by it's meta and value.
  @param {Object} fieldMeta - metadata of one field
  in FormGenerator format.
  @param {Object} value - { %fieldID%: %fieldValue% }
  @return {Object[]} - [ %failed_validators% ]