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).
Merge pull request #1063 from lucaswerkmeister/patch-1

Add .osm extension for XML
2.10.12
Merge pull request #1075 from github/release-2.10.12

cut a 2.10.12 release
rebase off of current master