Turtlebot stage simulation.

to simlulate simple 2d navigatin also test virtual sensor

how to use

```
> roslaunch turtlebot_stage turtlebot_in_stage.launch is_elastic_planner:=false
```

```
> roslaunch turtlebot_stage turtlebot_in_stage_virtual_sensor_enabled.launch is_elastic_planner:=false
```