extract selenium install logic to a module

[Sergeon]

Before all: thanks you for this cool repo. It helped me a lot configuring nightwatch in a real project 👍

Currently, the whole nightwatch.conf.js is set as a postinstall hook in package.json. But the only reason to do that -as far as I understand- is to ensure that selenium gets downloaded. This is pretty misleading since people just copypasting the configuration object from nightwatch.conf.js to their own config file as a template would not get the tests to run, since selenium would not be downloaded.

Also, this way is just more clear what the postinstall hook is about.

Since now both the config files and the selenium-install.js module need the BINPATH, constants were also refactored to a common module.

[Sergeon]

Soz! I checked for a contributing.md in this project, didn't know you guys have a global one. I'll check it and open the issue.