docs: replace all instances of node.js with io.js because trademark.

I doubt this is acceptable "as is" but it's a good starting point to talk about all the places the trademarked term "node.js" appears in the repo and what, in all, might need to be changed in order to remove it.
You'd have to take that up with [upstream](https://github.com/bestiejs/punycode.js), it's copied verbatim into the source tree.