Fix: secure cookies not working.

If you're behind a proxy -- such as nginx -- that terminates your SSL
connections, Express doesn't emit secure cookies.

To override this, we need to set proxy: true in the session settings.

We also might as well set 'trust proxy' in the global settings.

Links:

 - https://github.com/visionmedia/express/issues/967#issuecomment-12141099
 - http://stackoverflow.com/a/14473557/8446