Download selenium server

I modified the installer to do two things (separate commit)

First of all I made it easier to understand the different platforms and the url that is created.
Second I made it also download the selenium server that can be used for testing nodewebkit apps.

I realise that probably not everyone will want selenium server however they probably should test their apps and having it there will make it easier at least!

I am open to changing that if there is a good way of toggling it - not sure there is with npm?