* src/dynamic-graph-py.cc: rename plug -> w_plug,
	* src/dynamic_graph/__init__.py: bing wrap.w_plug to plug.

	  * src/CMakeLists.txt,
	  * src/dynamic-graph-py.cc,
	  * src/dynamic_graph/__init__.py,
	  * src/dynamic_graph/entity.py: new,
	  * src/dynamic_graph/signal_base.py: new,
	  * src/entity-py.cc: new,
	  * src/signal-base-py.cc: new.