Add Retina support to Piecon

Hi there,

Thanks very much for piecon, I love it. I've added Retina support, so it now looks gorgeous on the new Retina MacBook Pro. 

![Screen Shot 2012-12-23 at 13 01 12](https://f.cloud.github.com/assets/47539/28904/458ff436-4d01-11e2-8017-b95fdb713f67.png)

Many thanks,
James