Added support for executors

This pull request replaces the current Node.h with a larger interface which includes executors, groups, timers, and some other common utilities which are required to implement the `ros1_like` examples in https://github.com/ros2/examples/pull/1.
If it were a member variable, but it's a local scope. I would use a counting variable if it were stored in the class, which I think would be a good idea.