changed codes to be used in hydro

I changed spawn_objects.launch to make the gazebo-world like this page. 
https://code.google.com/p/rtm-ros-robotics/wiki/pr2_gazebo_sample

The picture shown below is the result.
![gazebo_pr2](https://cloud.githubusercontent.com/assets/7259700/4605919/4e4d259e-5204-11e4-8a74-e3b77725ac0b.png)