Review 'app.js' proposal for discussion

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#### DO NOT MERGE 
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This PR proposes some ideas for discussion as to how `app.js` should be implemented.

In previous versions of Mojito, `server.js` acted sort of a black box and it was not easy for users to hook handlers / middlewares on the `app` instance. 

The example `examples/newsboxes` has been updated and can be started with `node app.js`.