add support for refreshing plex library after conversion

Many Plex users make use of this script. Currently, due to the way it's run, sickbeard will only refresh the Plex library once when it has moved the file and not again when this script has finished its conversion. Because of this, Plex's knowledge of the file will be incorrect once the conversion is finished. This patch sends a refresh notification to Plex after the conversion is done to solve this issue.