* src/entity-py.cc: change std::string in char*.

    * src/entity-py.cc.

	  * src/dynamic-graph-py.cc,
	  * src/entity-py.cc.

    * src/dynamic_graph/entity.py,
    * src/entity-py.cc: the name of the entity class to be created is now given
      as input to the constructor of Entity python class.

    * src/entity-py.cc.

	  * 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.