doc: replace v8::Handle<T> with v8::Local<T>

v8::Handle is on its way out, to be replaced with v8::Local.  Update the
addons documentation accordingly.

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>