Make object detection possible in simulation.

This makes the 'sim' argument a parameter that can be passed in while launching PbD. This allows you to do object detection in simulation by enabling the simulated kinect. 

Running on the robot happens as usual. To run on the desktop, you can now do

```bash
$ roslaunch pr2_pbd_interaction pbd_demo_robot.launch sim:=true
```

and you get object detection.

As a note, my editor automatically strips whitespace from line endings, so that's what the other changed lines are.