Added leaflet.js option to check for retina screens and provide a nicer map.

Simply added 'detectRetina: true' to the tilesets loaded via Leaflet.

Before:

![Screen Shot 2013-03-10 at 18 47 27](https://f.cloud.github.com/assets/121439/241492/ffe589ca-89b2-11e2-8f36-11f94d9d904e.png)

After:

![Screen Shot 2013-03-10 at 18 47 03](https://f.cloud.github.com/assets/121439/241493/02cc3b70-89b3-11e2-955b-9770c753cb53.png)