gst-libs/gst/net/: Added buffer subclass to store extra to/from addresses for network sources/sinks.

Original commit message from CVS:
* gst-libs/gst/net/Makefile.am:
* gst-libs/gst/net/README:
* gst-libs/gst/net/gstnetbuffer.c: (gst_netbuffer_get_type),
(gst_netbuffer_class_init), (gst_netbuffer_init),
(gst_netbuffer_finalize), (gst_netbuffer_copy),
(gst_netbuffer_new), (gst_netaddress_set_ip4_address),
(gst_netaddress_set_ip6_address), (gst_netaddress_get_net_type),
(gst_netaddress_get_ip4_address), (gst_netaddress_get_ip6_address):
* gst-libs/gst/net/gstnetbuffer.h:
Added buffer subclass to store extra to/from addresses for
network sources/sinks.