Bundle all ChromeDrivers when publishing

Put ChromeDriver for Windows, Mac, and Linux into the build when publishing to NPM. The build is set at the moment to use ChromeDriver 2.8 (as I couldn't get the latest, 2.9, to reliably work).

Bundle will include

```
build/chromedriver/linux/chromedriver
build/chromedriver/mac/chromedriver
build/chromedriver/windows/chromedriver.exe
```

And `chrome.js` gets the correct version for the platform upon which it is run.