Ensure that things loaded by server/index.js override addons.

Express will always process the first matching middleware.

A couple examples where this is bad:

* If a proxy was setup, it is always hit before any http-mocks
  you have setup (since it is itself an addon).
* If you want to serve static content (or another express middleware)
  from `server/index.js` you could never handle requests with an accept
  header including HTML. If you did, it would have hit the history state
  addon and just served up `app/index.html`.

In general, it is our policy that the application will always "trump"
addon code.
Merge pull request #2008 from rwjblue/app-wins-express

Ensure that things loaded by server/index.js override addons.