Use footstep group coloring

* support coloring footstep based on jsk_footstep_msgs::Footstep::footstep_group
![screenshot from 2014-10-17 16 41 30](https://cloud.githubusercontent.com/assets/40454/4675873/6ce17696-55d2-11e4-99b1-ff28c027f4e1.png)