Add .osm extension for XML

The OpenStreetMap project (openstreetmap.org) uses the .osm extension for the OSM XML format (http://wiki.openstreetmap.org/wiki/OSM_XML).

Full “real-world” OSM files are usually too big to put in a repository, but small sample files may be used to test applications that process OSM files, for example [here](https://github.com/routeKIT/routeKIT/tree/master/test/edu/kit/pse/ws2013/routekit/precalculation).