Jest for unit tests

[Jest](http://facebook.github.io/jest/) has [good support for React](http://facebook.github.io/jest/docs/tutorial-react.html#content), so here's a feature for generating a project with Jest support out of the box. Tests are assumed to be in "__tests__" directories, like in the [flux-todomvc example](https://github.com/facebook/flux/tree/master/examples/flux-todomvc).
This is necessary because app.js is not used when testing and therefore React is not in scope through `window.React`.