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)


This module is specifically designed to be ROS independant as much as possible. Use of ros::Time was specifically designed out because it does not work in the case of a python wrapper. (ros::Time only works if roscpp is running. in pytf implementations ros::Time::now() does not work in simulated time) We need to find a way to implement this without adding that dependency back.