Add centroid coordinates to the popup for use in the Wikipedia template

When an article is missing the coordinates in Wikipedia the popup currently asks to add the popup and describes the {{Coord}} template to add generically.

This pull request add an osm_centroids.py script that creates a Spatialite (SQLite) database with the centroids of all elements (nodes, ways, relations). These information are then passed to the missing element and the showed in the popup so that the template can be copy-pasted.

Here's an example of the new popup:
http://imageshack.com/a/img827/6530/mp4o.png

(compare with the old popup: http://imageshack.com/a/img843/1999/xfov.png)