Feature/implement app settings in browser

- Exclude express-middleware from browser bundle to fix a regression introduced by "#293 Upgrade to Express 4.x"
- Add `test/app.test.js` to the list of unit tests run by karma
- Make `app.get` and `app.set` available in the browser.

`app.get` and `app.set` are used by loopback-boot to configure app settings from `app.json`.

/to @ritch please review
/cc @raymondfeng