node: rename from io.js to node

This commit replaces instances of io.js with Node.js, based on the
recent convergence. There are some remaining instances of io.js,
related to build and the installer.

Fixes: https://github.com/nodejs/node/issues/2361
PR-URL: https://github.com/nodejs/node/pull/2367


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>