add a bunyan serializer for supporting context objects

[DonutEspresso]

FYI @yunong @micahr supports the new context objects. Might look something like:

[2016-02-08T23:24:09.291Z] ERROR: unit-test/49005 on lgml-aliu: wrapped error (err.code=InternalServer)
    InternalServerError: ISE (foo=bar, baz=1)
        at Context.<anonymous> (/Users/aliu/Sandbox/npm/restify-errors/test/index.js:743:25)
        at callFnAsync (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runnable.js:306:8)
        at Test.Runnable.run (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runnable.js:261:7)
        at Runner.runTest (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:421:10)
        at /Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:528:12
        at next (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:341:14)
        at /Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:351:7
        at next (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:283:14)
        at Immediate._onImmediate (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:319:5)
        at processImmediate [as _immediateCallback] (timers.js:383:17)
    Caused by: Error: boom
        at Context.<anonymous> (/Users/aliu/Sandbox/npm/restify-errors/test/index.js:742:23)
        at callFnAsync (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runnable.js:306:8)
        at Test.Runnable.run (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runnable.js:261:7)
        at Runner.runTest (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:421:10)
        at /Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:528:12
        at next (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:341:14)
        at /Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:351:7
        at next (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:283:14)
        at Immediate._onImmediate (/Users/aliu/Sandbox/npm/restify-errors/node_modules/mocha/lib/runner.js:319:5)
        at processImmediate [as _immediateCallback] (timers.js:383:1

[micahr]

Looks great! Can't wait to make use of this.

[yunong]

LGTM, ship it!