* initial commit for rclcpp::time::now()
* switch between times
* introduce time class
* add rclcpp/time.hpp to rclcpp/rclcpp.hpp
* throw exceptions on time error
* fix test_time to catch exceptions
* explicit one-parameter constructor
* fix msvc compiler warnings
* address review comments
* cleanup includes
* re-add todo for fixing test once ros-time is there