Child logging

Appends a unique requestId to each express call, and exposes a child logger to the req object.
As there is no standard now, the name should be ok, but I preferred 'req_id' just because it suggested in bunyan's document :-)

And another thing is flexibility may need to provided so people can enable/disable(which is enabled by default) the request id or integrated with the middleware that they already used.

It will similar to the `jar` option in [request](mikeal/request), it can be boolean or object. here an option can be a boolean or function to get the request id from `req`.