gst-libs/gst/rtp/: Added rtp payloader base class.

gst-libs/gst/rtp/: Added rtp payloader base class.

Original commit message from CVS:
* gst-libs/gst/rtp/Makefile.am:
* gst-libs/gst/rtp/gstbasertppayload.c:
(gst_basertppayload_get_type), (gst_basertppayload_base_init),
(gst_basertppayload_class_init), (gst_basertppayload_init),
(gst_basertppayload_finalize), (gst_basertppayload_setcaps),
(gst_basertppayload_chain), (gst_basertppayload_set_options),
(gst_basertppayload_set_outcaps), (gst_basertppayload_push),
(gst_basertppayload_set_property),
(gst_basertppayload_get_property),
(gst_basertppayload_change_state):
* gst-libs/gst/rtp/gstbasertppayload.h:
Added rtp payloader base class.