New elements interleave and deinterleave, implement channel interleaving and deinterl...

New elements interleave and deinterleave, implement channel interleaving and deinterleaving.

Original commit message from CVS:
2007-01-07  Andy Wingo  <wingo@pobox.com>

* configure.ac:
* gst/interleave/Makefile.am:
* gst/interleave/plugin.h:
* gst/interleave/plugin.c:
* gst/interleave/interleave.c:
* gst/interleave/deinterleave.c: New elements interleave and
deinterleave, implement channel interleaving and deinterleaving.
The interleaver can operate in pull or push mode but the
deinterleaver is more like a demuxer and can only operate in push
mode.