node: add AsyncListener support

The AsyncWrap class has been added as a base class for everything that
will be performing an asynchronous request. This is meant to be a
simpler way to track when requests have gone out, and to allow users
hooks to know when this happens.

Currently only ReqWrap and HandleWrap are covered. Full support will be
added in the future for all the *Wrap classes.

This comes with a new API:

* process.createAsyncListener(listener, callbacks, domain): This will
  create an AsyncListener instance that can be added and removed when
  necessary. The "domain" passed will propagate to each callback unless
  the listener at some point returns a new object.
  Returns the new AsyncListener instance.

* process.addAsyncListener(listener, callbacks, domain): Will create a
  new AsyncListener instance and immediately add it to the listening
  queue.
  Returns the new AsyncListener instance.

* process.removeAsyncListener(instance): Remove an AsycListener instance
  from the current queue and stack.
  Returns undefined.

Explanations for each callback:

* listener(): Callback that's called immediately when a *Wrap, Timer or
  nextTick is queued.

* callbacks.before(context, domain): Run immediately before the async
  callback is about to run. "context" is the "this" of the caller,
  except in the case of nextTick. Where the callback object is passed
  instead. "domain" is the object passed to createAsyncListener() or
  returned from listener().

* callbacks.after(context, domain): Same as before(), except run
  immediately after the callback has run. Except if the callback has
  thrown, in which case this will not be called.

* callbacks.error(domain, error): Called when a callback has thrown.
  Currently the "context" is lost in this case so it's not passed, but
  the "domain" object still is. If the error is properly handled the
  return true to continue program execution. If multiple error handlers
  are queued all will be called, and if any one returns true then the
  error is assumed to have been handled.

In the case of callbacks, they are only allowed to throw two levels
deep. Afterwards the error handler will always return false. This is to
prevent infinite recursive throws.

As you'll see, the domain module still exists. Full support has been
added in it's previous form until a complete solution is patched. The
goal is to have the domain module use AsyncListeners and be as far
removed from core as possible. When that day comes, I'll fully enjoy
torching every last line of integrated code until it's all burnt
stubble.
async-wrap: integrate with WeakObject

Making WeakObject inherit from AsyncWrap allows us to peak into almost
all the MakeCallback calls in Node internals.
async-wrap: add prototype methods to udp/tcp/pipe

Now it's possible to add/remove an async listener to an individual
handle created by UDP, TCP or Pipe.
crypto: make RandomBytesRequest a class

Since RandomBytesRequest make a call to MakeCallback, I needed it to be
a class so AsyncWrap could handle any async listeners.

Also added a simple test for an issue had during implementation where
the memory was being released and returned.
crypto: convert pbkdf2_req to a class

pbkdf2_req has been renamed to PBKDF2Request and converted to a class.
It now uses AsyncWrap::MakeCallback.

Also includes, using env()->ondone_string() instead of "ondone" and
using malloc instead of new char[].
cares: add AsyncListener support
timers: add/remove listener after creation

To accomplish the after callback running in a setInterval, when
clearInterval was run within the callback, the method is run inside a
process.nextTick.
test: update to check add/remove on a handle
domain: remove domain code from src/

Much of domain functionality can be achieved by async listeners, so they
have been refactored to use them.

There are two cases (one in net.js and other in _http_client.js) where
the handle isn't created until after the instantiation of the event. So
it was necessary to add a check if domains exist, then attach the
domain's listener to the handle at time of creation.

Because the async listener handles errors at the point of asynchronous
entry, the need to enter/exit a domain in the event emitter was no
longer needed.
doc: add some info about AsyncListeners

More information and some examples are necessary.
domain: attempt to more properly remove domain
fixes: WIP from latest review
weak-object: Dispose() on self

Also included assert on AsyncWrap that makes sure that
persistent().IsEmpty().
SQUASH: fix indutny review
SQUASH: few misc things that need to be squashed
WIP: fix errorHandler so exit event fires
SQUASH: change how error handling works
test: add additional async listener tests
SQUASH: fixed up my own tests
SQUASH: fix forgotten adjustment to queue length

asyncFlags[kCount] wasn't updated after an AsyncListener was removed
from the asyncQueue. This caused runAsyncQueue to continue to be called
regardless of there being no instances in the queue.