Added tsstamper element

The purpose of the tsstamper element is to add PTS to each buffer
based upon extracted PCR.  This is to provide means of
synchronization when sending out MPEG-TS over e.g. UDP.

In particular this should enable using GStreamer to generate
multicast streams.
gsttsstamper: Replace explicit fprintf() with GST_DEBUG macro
tsstamper: Handling new segments

As the tsstamper sets PTS on each buffer, a downstream element might expect
segment events to be in time format.  To cater for this the tsstamper has
to swallow all segement events and produce its own based on the time
information in the first buffer.