Fix config.json and add favicon support

/to @bajtos or @ritch 

The PR fixes 3 issues:
- Make sure the app is listening on 0.0.0.0:3000 instead of localhost:3000
- Add favicon middleware to avoid 404 warnings
- Fixes https://github.com/strongloop/generator-loopback/issues/12
@sam-github this is the reason why you saw incorrect URL printed by the app run via `PORT=12345 node app`.

When there is not `url` option provided by `config.json`, LoopBack will built it in a `listening` handler using the actual port of the server. (This means you can listen on port 0 and still get a valid URL.)

However, when the URL is provided by the config, loopback does not change it.

I am proposing to change the scaffolded `config.json` to `Option B` as presented above - i.e. omit both `host` and `url` values.

Alternatively, we can modify the code building the URL in loopback to replace `0.0.0.0` with `localhost` in the URL, so that `config.json` can include `host: 0.0.0.0` and omit the `url` option.

@raymondfeng @rmg thoughts?