Geocoding widget

![image](https://f.cloud.github.com/assets/924374/1168941/11b7c70c-20a9-11e3-8e8a-32f69be89296.png)

There's a lot more I could do here, but I think it's reasonable to merge it into master in its current state.

I suspect the most controversial thing here is the location of the new widget in the `Viewer`.  I originally had it in the top left, but @emackey made the excellent point that we should reserve that for app-specific functionality.

This branch includes all the `bingKey` changes as well, so the diff will get simpler once that branch is merged.