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 
Let's rename this to version. I'm not a fan of obscure variable names as it makes it hard to understand what it is at it's point of usage.