Implement the category of hSets, and of hProps
Use issig rather than _sig or equiv_sig
Add more namespacing to set category functors

To me, [SetCategory.Functors.to_set] seems nicer than
[SetCategory.to_set] to refer to the type of functors from a category
[C] to [Set].
Merge pull request #230 from JasonGross/set-category

Set category