Preview support

Fill the harvested resources `preview_url` with the CKAN embedded preview (See http://docs.ckan.org/en/latest/maintaining/data-viewer.html, http://docs.ckan.org/en/ckan-2.2.3/data-viewer.html#embed-previews and https://github.com/ckan/ckan/wiki/Resource-Views)

See opendatateam/udata#1564 for the complete details