New map UI

This isn't quite ready to go (waiting on an upstream [Leaflet bug](https://github.com/Leaflet/Leaflet/pull/1846)), but I wanted to share and possibly get deployed for testing.

![image](https://f.cloud.github.com/assets/98601/765441/ff7a939a-e82b-11e2-8821-8874aa6c99ba.png)

Notable features:

* New layer chooser UI
* New geolocation control
* New share control (with room for "Include marker" checkbox and reorganization/consolidation with some of the "Export" features)
* Consistent style with map UI on other pages (just the zoom buttons)

Based on [work](https://github.com/samanpwbb/osm-redesign/blob/gh-pages/renders/layer-switcher.png) by @samanpwbb.