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.
Because it's easier this way.  The build step that bakes the files in lib/ into the binary isn't ready to pull in files from elsewhere.  Because punycode.js is the only externally developed JS library, it's easier to have it live in the lib/ directory.