DeterminesWorkKind takes the work instance now

This will make it easier to determine kind based on other attributes.

Also refactored the spec, (hopefully) to be clearer and easier to
extend.
Generalize DeterminesWorkKind::Classifier

Mainly because we'll need to call it against non-url strings soon.
Determine kind by description or .rar extension
Use eq, not be in spec
Classify photos/images, consistent regex literal
Small refactor

* Rename method, add private