mpegtssection: Include a timestamp

Submitted by Brendan Long

Link to original bug (#729134)

Description

I'm looking at exposing MPEG-TS sections as metadata in HTML5, and it would be nice if we had consistent timing information. GST_MESSAGE_TIMESTAMP() should give us a time, but it's always invalid for MPEG-TS section messages.

The only timestamp I see is MpegTSPacketizer2.last_in_time. Would that be a reasonable time to use for the message?