For shapefile export, tell ogr2ogr to promote geometries to multi-*

[prashtx]

Right now we assume there won't be a mix of Point with MultiPoint geometries, Polygon with MultiPolygon, LineString with MultiLineString. That might not always be true, though. We should be able to promote to multi-geometries with -nlt PROMOTE_TO_MULTI (http://www.gdal.org/ogr2ogr.html).