Access current time from Node

In ROS2 time keeping is the responsibility of Node. We recently added getClock() to Node. In addition to accessing the clock is convenient access of the current time. This issue proposes adding `now(): Time` method to Node similar to the now() method of the [rclcpp.Node](https://github.com/ros2/rclcpp/blob/master/rclcpp/src/rclcpp/node.cpp) class. 