Add childLogger middleware.

This PR add a `childLogger` middleware. This middleware attaches a child logger to every request object with `req_id` set to uniq id so you can further access it with `req.log`, example:

```javascript
app.get('/test', function(req,res) {
  req.log.debug('Hello, world');
  res.send('Hello');
});
```

Useful for debugging. It is described in "Recommended/Best Practice Fields" of [bunyan](https://github.com/trentm/node-bunyan)