Rewrite the introductions to jsdom.env/jsdom.jsdom.
Remove `document.createWindow` function.

It only exists for backward-compat, and this is a big compat-breaking release.
Remove `jsdom.html`.

It is doing the job that `jsdom.jsdom` should be doing.
Remove `jsdom.version`.

Just use `require("jsdom/package.json").version` instead, like you would for every other Node package.
Hide `level` setting better.

It is now a config option, not an important argument. This changes the signature of `jsdom.jsdom`, and removes `jsdom.level`.
Better handling missing <html>/<body>/<head>.

This still isn't the actual HTML parser algorithm; for that we'll need a better parser, that correctly creates those elements when they are missing. But now we make a much better attempt to ensure the <html><head></head><body></body></html> structure.

You can also now pass an empty string to `jsdom.env` and it will create an empty HTML document.
Fetch/process external scripts by default in `jsdom.env`.

@tmpvar, do you think this is a good idea?