AsyncListener is a JS API that works in tandem with the AsyncWrap class
to allow the user to be alerted to key events in the life cycle of an
asynchronous event. The AsyncWrap class has its own MakeCallback
implementation that core will be migrated to use, and uses state sharing
techniques to allow quicker communication between JS and C++ whether the
async event callbacks need to be called.

Making WeakObject inherit from AsyncWrap allows us to peak into almost
all the MakeCallback calls in Node internals.

Now it's possible to add/remove an async listener to an individual
handle created by UDP, TCP, Pipe or Timer.

Since RandomBytesRequest makes a call to MakeCallback, 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.

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[].

The domain module has been switched over to use the domain module API as
much as currently possible. There are still some hooks in the
EventEmitter, but hopefully we can remove those in the future.

Documentation has been added on how to use the AsyncListener API.