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.

Instead use the template functions in util.h.

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.

Make the pointer-to-wrap const (i.e. mutable but not assignable) to
prevent accidental reassignment in the macro body.

The check has virtually zero overhead and it simplifies the call sites
because they were calling IsEmpty() anwyay.

The handle object was checked against the wrong constructor template.
Put another way, it was unwrapped as the wrong StreamWrap type.

This is a big commit that touches just about every file in the src/
directory. The V8 API has changed in significant ways. The most
important changes are:

* Binding functions take a const v8::FunctionCallbackInfo<T>& argument
  rather than a const v8::Arguments& argument.

* Binding functions return void rather than v8::Handle<v8::Value>. The
  return value is returned with the args.GetReturnValue().Set() family
  of functions.

* v8::Persistent<T> no longer derives from v8::Handle<T> and no longer
  allows you to directly dereference the object that the persistent
  handle points to. This means that the common pattern of caching
  oft-used JS values in a persistent handle no longer quite works,
  you first need to reconstruct a v8::Local<T> from the persistent
  handle with the Local<T>::New(isolate, persistent) factory method.

A handful of (internal) convenience classes and functions have been
added to make dealing with the new API a little easier.

The most visible one is node::Cached<T>, which wraps a v8::Persistent<T>
with some template sugar. It can hold arbitrary types but so far it's
exclusively used for v8::Strings (which was by far the most commonly
cached handle type.)

fallout from the tls_wrap feature landing

Originally contributed by @tjfontaine, but modified to be faster and
more generic.