Add validators for ISNI (c.f. ORCID) and InSet.

Add new validators for ISNI and fixed Sets; fix comment on Date validator.  These validators will be used in SUSHI-Lite, but are generic enough to be global.
We can implement stricter checking with the third optional $strict parameter of in_array(), but then this is no longer backwards compatible with the former code of classes/form/validation/FormValidatorInSet.inc.php:46.

I like the stricter checking option, but could also just change the test cases.

Any preference?