[jsk_rviz_plugins] Diagnostics plugin

show diagnostics with circle in SF-way

* the color changes according to the status of diagnostics
* it seems to be cool

![screenshot from 2014-05-07 12 35 27](https://cloud.githubusercontent.com/assets/40454/2898275/88385974-d598-11e3-926e-9ec349a70adb.png)