display difference between reference clock and each tf node, which was supported until groovy

frame_view until groovy shows reference clock and how much each node in tf tree differ from that[1], which is very convenient when we use ros over multiple machines and find out which machine needs to be ntpdate-ed. but from hydro, that information is not displayed[2].

This patch shows that information, but not work for simulation environment which we need to read '/clock' and if I understand correctly the tf2 is designed not to depend on ROS communication framework. But still this helps when we working on real robot.

[1]
![screenshot from 2014-12-06 13 40 10](https://cloud.githubusercontent.com/assets/493276/5326217/5cdc9bd8-7d4e-11e4-840e-e3601aa98a4d.png)

[2]
![screenshot from 2014-12-06 13 42 16](https://cloud.githubusercontent.com/assets/493276/5326216/4ca0313a-7d4e-11e4-893c-0d8fe4a29428.png)


I think the best way would be to provide a second version of the function where you pass in the current time. If the new API is called it renders it with the time delta. If you use the old API it renders without the extra information. And then the higher level process(aka view_frames) will pass in ros::Time::now() to the BufferCore instance.