Expose language setup pages as public routes.

```
+ Add setup.rb route handler. '/setup/:language/'
  routes display setup information for each
  supported language, even if the user has
  already started the language track.
+ Add setup handler tests.
```

This is a simple handler that displays the setup pages for each language, which currently disappear once a user starts a language track. It would be useful to make these public, both in case users need them for future reference, and so the Exercism gem can refer users whose test suites error due to misconfiguration to a friendly help page.