PR for parents() function

Changed the `.parents()` method to work like jQuery's.

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Also moved the existing `parents()` function and renamed it to `closest()` ... I note that @vivekimsit already created a PR for a `closest()` method, so a bit confusing... but I think his method closesly matches jQuery's one which seems sensible.

Also did the unit tests and documentation alphabetically, and noticed that `.add()`'s unit tests where out of place, so moved them, too.

Cheers, Si. 👊