Added /auth/login which recognizes an optional `return_to` param.
Added /auth/login which recognizes an optional `return_to` param.

The params indicates a different URL to be redirected after login. The existing `/auth/heroku` always redirects to `request.url`, which makes sense if the authentication is enforced but is not really flexible for other use cases.

recurser/heroku-bouncer,heroku/heroku-bouncer,cbartlett/heroku-bouncer