By adding support for an environment variable `PUPPETEER_DOWNLOAD_PATH` it is possible to support downloading the browser binaries into a folder outside the `node_modules` folder. This makes it possible to preserve previously downloaded binaries in order to skip downloading them again.
This patch refactors Puppeteer-Firefox code to declare public
API in `/lib/api.js` and use it to setup async stack hooks
over the public API method calls.
