Fix: initial import of country_osm_grid.sql.gz

[Piskvor]

Unless the country grid is downloaded before setting up Nominatim, the setup will fail: Error: you need to download the country_osm_grid first: wget -O /app/git/data/country_osm_grid.sql.gz http://www.nominatim.org/data/ The command '/bin/sh -c sudo -u postgres /usr/lib/postgresql/9.3/bin/pg_ctl star hreads 2' returned a non-zero code: 1

This PR adds a command to download this file, which allows the build to complete succcessfully.

Also, it adds a --timestamping option to the wget requests, so that unchanged files are not pointlessly redownloaded (only files that have been modified are downloaded again).