bounding box in observation search responses

[kueda]

In response to http://api.inaturalist.org/v1/observations we'd get something like

{
  total_results: 4131950,
  page: 1,
  per_page: 30,
  bounding_box: {
    nelng: 1,
    nelat: 1,
    swlng: 0,
    swlat: 0
  },
  results: [
    ...
  ]
}

This would let us do some thing things like zoom the map appropriately on the taxon page.