This is another step towards making Puppeteer agnostic of environment
and being able to run in Node or a browser.
The files in the `node` directory are ones that would only be needed in
the Node build - e.g. the code that downloads and launches a local
browser instance.
The long term vision here is to have three folders:
* node - Node only code
* web - Web only code
* common - code that is shared
But rather than do that in one PR I'm going to split it up to make it
easier to review and deal with.
