Make gush viz platform independent

I tried to run ```gush viz``` on my shiny mac, and I got ```Unknown command 'xdg-run'```
![you_make_bunny_cry](https://cloud.githubusercontent.com/assets/185015/2553723/aac235e6-b6a8-11e3-8155-ce2c3df3aa94.jpg)


So, I changed hardcoded ```xdg-open``` with Launchy gem, plus used more flexible approach with getting tmp dir.