Redux: Convert test suite to Jest

Since we now recommend using Jest, we should practice what we preach 😄

Jest 15 introduces some major improvements and makes the TDD process a heck of a lot easier and more fun. It's also faster and and has more helpful error messages. It's really awesome and there's good reason for its recommendation in the Writing Tests docs.

This is a request for a PR from the community. If you want to help out with open source development, this is a great chance to do so. Just remove mocha (the current test runner) and expect (Jest provides an expect function built-in), and switch out the test suite to run through Jest instead. There are hopefully only minimal changes needed to the actual test code, so this should mostly be just simple infrastructure work.

It _should_ be good for beginners, but let us know if anything is particularly challenging. We can pass that along to the Jest team, who is always looking for ways to make the tool as friendly and fun as possible.