added media:thumbnail to the MRSS feed
Created by: abrahammartin
- Modified the export_sms_feed.sh to use the media image_id or in its absence its collection image_id
- Added a new command line parameter for "base-image-url" that will be used as the base URL where the images are being served (same as "base" for videos)
Closes https://github.com/uisautomation/sms-webapp/issues/36