$ git commit -m 'handle logout properly

- make the logout endpoint a PUT
- call flask_login.logout() on logout
- send no-cache headers on the app page, forcing users to
  log back in rather than hit the cached version of the app
Slap a logout button on the app page

- the button hits the /logout end-point on the backend and redirects to
  the welcome page
Merge pull request #34 from waltaskew/feature/logging-out

In which we slap a logout button on the app page