process.addAsyncListener polyfill

This is an implementation of Trevor Norris's
process.{addAsyncListener,removeAsyncListener} API for adding behavior to async
calls. You can see his implementation (currently a work in progress) on
Node.js core pull request #6011.
This polyfill / shim is intended for use in versions of Node prior to whatever
version of Node in which Trevor's changes finally land (anticipated at the time
of this writing as 0.11.7).

Here's his documentation of the intended API, which will probably get cleaned up
here later:

createAsyncListener(callbacks[, initialStorage])

* callbacks {Object}
* initialStorage {Value}

Returns a constructed AsyncListener object. Which can then be passed to
process.addAsyncListener() and process.removeAsyncListener(). Each
function parameter is as follows:

1. callbacks: An Object which may contain four optional fields:
* create: A function (storage) that is called when an asynchronous event
is queued. Recives the storage attached to the listener. storage can be
created by passing an initialStorage argument during construction, or by
returning a Value from create which will be attached to the listener
and overwrite the initialStorage.
* before: A function (context, storage) that is called immediately
before the asynchronous callback is about to run. It will be passed both
the context (i.e. this) of the calling function and the storage.
* after: A function (context, storage) called immediately after the
asynchronous event's callback is run. Note that if the event's callback
throws during execution this will not be called.
* error: A function (storage, error) called if the event's callback
threw. If error returns true then Node will assume the error has been
properly handled and resume execution normally.
1. initialStorage: A Value (i.e. anything) that will be, by default,
attached to all new event instances. This will be overwritten if a Value
is returned by create().

addAsyncListener(callbacks[, initialStorage])

addAsyncListener(asyncListener)

Returns a constructed AsyncListener object and immediately adds it to the
listening queue.

removeAsyncListener(asyncListener)

Removes the asyncListener from the listening queue.