Add Retina Support

Hello! I have a change for Deluge.app that enables Retina support.

I picked up a solution from [SO](http://stackoverflow.com/a/11798968), which works wonderfully. Image comparison before and after:

![default](https://f.cloud.github.com/assets/68210/30574/cbfa76c8-4ebd-11e2-887a-72320bd49d6b.png)

![retina](https://f.cloud.github.com/assets/68210/30575/cf56577e-4ebd-11e2-91c1-bcc9b08e9e56.png)

This can also be done "in-place" to existing copies, however having it as a default would be great.