fix #86, retriggering of assets without leaving and re-entering their range.  modify_stream now only refreshes the available recordings if tag parameters are present in the request.  If only location (lat/lon) are provided, recordings are not refreshed, thereby avoiding retriggering.  I also re-enabled the move_listener API call which can now be used alternatively to modify_stream for situations where only the location is being updated.  This should be reconsidered when a new API version is developed as there is some overlap now.