Fit map zoom and center to given bounds

Screenshot of code in action:

![sheely](https://cloud.githubusercontent.com/assets/33697/2937733/dc1957a6-d8cb-11e3-8bf4-cc22cc652d82.png)

I'm using my Shapely library in this example to perform a "sloppy" 100 meter buffer on a GeoJSON polygon and then fit the map to the buffered extents.

I notice that until the map is rendered via `display(m)` the model's `_south`, `_west`, `_north`, and `_east` have only the default values. I'd like to be able to fit to bounds without having to display first, but I'm not sure how to do that yet.