Add code to create maps for tags.

For a long time we had the capability to create overview maps for keys, but
never for tags (ie key-value-combinations). This commit now adds code to
create maps for frequently used tags. We can't create maps for all tags,
because each map takes about 8k in RAM and there are a lot of tags.
Remove old API versions (2 and 3).

The API v4 has been available since January 2013. Enough time to switch over.
Add API call to get maps of tag distributions.
Allow spaces in key-value geodistribution lst.