doc: don't use `using namespace v8`

Wholesale importing an entire namespace with `using namespace` is a bad
practice.  Remove it from the addons documentation and replace it with
proper `using` directives.  Wrap code in a namespace while we are here.

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>