add support for country selector

[critzo]

While the default for NDT tests is to use the "nearest" server provided by the Locate API v2, selecting a "nearest" server within a specified country is supported by the API as well. For people who may be geolocated incorrectly, or for those who may wish to test connectivity to a server further away, add support to select a country for this test, and use it with the request to Locate.

[stephen-soltesz]

Depends on: m-lab/ndt7-js#38