Fixups for categoricals

This PR will collect the various fixups for Categorical.

See #8078 for the ToDos.

Closes: #8077 -> API/FIX: wrap result cut/qut in Series and make it ordered
Closes: #8074 -> Discussion
Closes: #8078 -> Todos

This has a (passing) unittest for #8138 but no code changes. Not sure what was the change which fixed the bug there...
maybe add a link to this section from http://pandas-docs.github.io/pandas-docs-travis/comparison_with_r.html (maybe separate PR)