Update Nightwatch 

This update will make Nightwatch 
- download the selenium web driver and the chrome web driver on install
- puts the downloaded file in the right place with a generic filename

Note:
config.js and install.js is a straight copy from the selenium-standalone npm module

Reviewer: @scalvert 
Can we rename this to version while we're at it. Also, let's extract the version number out to a variable similar to the selenium one. Let's rename both to the appropriate names, i.e. seleniumVersion and chromeDriverVersion.