Add a basic demo crunchy app

[nealrichardson]

Use the common crunch-test fixtures for a backend. Something simple enough that illustrates how to set up a crunchy app (using shinyDataset and whatever else) to give our users a reasonable starting point. Presumably we could also use it for smoke testing that we haven't broken anything in the code (though this won't give us any test coverage for auth, which is one of the bigger challenges).