Update links.

"You're Missing the Point of Promises" was pointing to the gist version, not the blog post version. And the Promises/A+ spec link was pointing to the old GitHub pages URL, not the top-level domain.
Recognize that Mocha now allows promise-returning tests!

As such, I reordered the documentation to treat promise-returning test runners as the norm, and others as deviants that need accomodation. I also expanded on the list of frameworks in need of reeducation, and added pointers to ones that work already.
Reorder sections.

Basically, "Working with Non-Promise–Friendly Test Runners" should be at the bottom.
Un-comment out some tests.

These were presumably commented out during the 4.0.0 rewrite and I forgot to put them back.
Switch to Istanbul for code coverage.

It seems much better maintained.