Fixes #22

Conflicts:

	include/dynamic_graph_bridge/ros_interpreter.hh
	src/ros_interpreter.cpp

The old names are kept for now, but will disappear in the
future.

The old names are kept for now, but will disappear in the
future.

  Define entity RosTime that exports Ros time in a signal,
  add this entity in class dynamic_graph.ros.ros.Ros.
  Transfer Ros time in signal of type ptime instead of vector.