export the bunyan serializers

[trentm]

Before restify-clients was split from restify, one would get access to Restify's Bunyan reserializers (especially its client_req and client_res serializers) via:

var restify = require('restify');
var log = bunyan.createLogger({
    ...
    serializers: restify.bunyan.serializers
});

This is required to create a Bunyan logger for restify clients so that you don't get big garbage in trace-level logs when client_req and/or client_res is logged (e.g. here:

clients/lib/HttpClient.js

Line 314 in 8765244

log.trace({client_req: opts}, 'request sent');

).

There is some history and subtlety here:

• If you pass a Bunyan logger with no serializers to the restify clients, then it will add the necessarily serializers:

  clients/lib/HttpClient.js

  Lines 401 to 405 in 8765244

  	if (!this.log.serializers) {
  	// Ensure logger has a reasonable serializer for `client_res`
  	// and `client_req` logged in this module.
  	this.log = this.log.child({serializers: serializers});
  	}

  However that code is insufficient if your logger has other serializers on it.
• That limitation was due to: fix empty serializers node-restify#501

However, even if that case is fixed (I'll try to do a PR for that), I think it would be good citizenship for restify-clients to export .bunyan.serializers for users that want to control their Bunyan logger serializers carefully. I'd be careful to NOT export the RequestCaptureStream class that was carried over from restify (the server) itself because it isn't used or needed in the client.

[trentm]

see #80 to correct a mistake made in this commit