doc: remove uses of v8::Isolate::GetCurrent()

v8::Isolate::GetCurrent() is slated for deprecation.  Replace its uses
in the addons documentation with v8::Object::GetIsolate(), etc.

PR-URL: https://github.com/iojs/io.js/pull/1125


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>