Added boundary boxes

Data from http://wiki.openstreetmap.org/wiki/User:Ewmjc/Country_bounds, except for boundary box for US which I added manually (original data seemed to have a bug for US). Need to add attribution to be complaint with [CC Attribution-ShareAlike 2.0](https://creativecommons.org/licenses/by-sa/2.0/) license (didn't do myself yet because I wasn't sure where to include that info, but can update PR if you provide instructions).