RSpec setup, specs for Auth model and App API.

Something to get started with. This covers:

* Model specs (unit / integration tests),
* API specs (where you directly ask, say, `/apps/index.json` for data and check the result), and
* An example request spec (where you drive a browser to interact with UI elements, eg. forms).

To run the specs, type `bundle exec rspec spec`. (We can add a `rake spec` task in the future if this gets annoying to type.)