Make logout configurable

For apps using heroku-bouncer, logout is best achieved via hitting '/auth/sso_logout'.

This PR attempts to allow us to override the logout URI versus configuration.

Note - I'm not sure the browserify bits of this have worked as they should, probably due to some config thing.  Can someone take a look and verify this isn't broken? I've been unable to test this as I couldn't compile it all the way.

> Running "browserify:basic" (browserify) task
Warning: module "domready" not found from "/Users/neil/code/fuji.js/public/fuji-pre-browserify.js" Use --force to continue.

> Aborted due to warnings.