add mutant logo with simple retina support

I've tried to accomplish something nice with pure css, but what the hell... 
you know how different all the browsers are...

So I made a clean image logo. 

![screen](https://f.cloud.github.com/assets/685742/1829845/8a4c7ad6-7307-11e3-83d4-f5cebca5545b.png)