Upgrade Icons for Retina Support

Hey there @joelfillmore, I'm using a Retina MBP and so I liked this theme, but the icons were all blurry. I've take some time and added 2x versions as close as I could make them to the originals.

Here's a link to my repo that has the vector versions if you want other changes to be based off of them: [rcdilorenzo/flatland-dark-vector-icons](https://github.com/rcdilorenzo/flatland-dark-vector-icons).