pipeline: g-i: allow clock to be NULL in gst_pipeline_use_clock()

https://bugzilla.gnome.org/show_bug.cgi?id=705751
buffer: Add annotations and pygi friendly extraction function

API: gst_buffer_extract_dup

Conflicts:
	gst/gstbuffer.h
buffer: fix Since: marker for new gst_buffer_extract_dup()