BaseObject is a class that just handles the Persistent handle attached
to the class instance.

This also removed WeakObject. Reordering the inheritance chain helps
prevent unneeded calls on instances that don't call MakeCallback.

env->context() may or may not create a new Local.  It currently does
not but don't depend on that behavior, create a HandleScope first.

Create a HandleScope before calling the Environment::GetCurrent() that
takes a v8::Isolate* as an argument because it creates a handle with
the call to v8::Isolate::CurrentContext().

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

Going back to the original names of Wrap/Unwrap now that most all the
class members that duplicate the name and functionality have been
removed.

Switch out to use UnwrapObject from util.h.

Drop the ObjectWrap dependency in favor of an internal WeakObject class.

Let's us stop worrying about API and ABI compatibility when making
changes to the way node.js deals with weakly persistent handles
internally.

This commit makes it possible to use multiple V8 execution contexts
within a single event loop.  Put another way, handle and request wrap
objects now "remember" the context they belong to and switch back to
that context when the time comes to call into JS land.

This could have been done in a quick and hacky way by calling
v8::Object::GetCreationContext() on the wrap object right before
making a callback but that leaves a fairly wide margin for bugs.

Instead, we make the context explicit through a new Environment class
that encapsulates everything (or almost everything) that belongs to
the context.  Variables that used to be a static or a global are now
members of the aforementioned class.  An additional benefit is that
this approach should make it relatively straightforward to add full
isolate support in due course.

There is no JavaScript API yet but that will be added in the near
future.

This work was graciously sponsored by GitHub, Inc.