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

Simply added 'detectRetina: true' to the tilesets loaded via Leaflet.

This just loads more tiles in the same place on retina screens. Not sure if these screenshots will come out nicely, but the difference is significant.

Seems that the Google Maps do this automatically.

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)