add roscore_check

Just show the current time unless roscore die.
Very simple, so maybe not useful. Please reject if then.
![screenshot_from_2014-12-23 10 07 29](https://cloud.githubusercontent.com/assets/3803922/5533469/0cc3d6a2-8a8c-11e4-9a8b-af7b4a9e7684.png)