[face_detector] Add RGB sample with PR2

[130s]

Add a face detection sample launch using RGBD topics with PR2.

• To run: simply roslaunch face_detector facedetector_rgb_pr2.launch.
• To see the face detection result, for instance using a nice jsk_rviz_plugins:
  
• Originally PR was opened toward pr2_apps but there I was suggested to add this in face_detector

[DLu]

It doesn't fit in the teleop package, but I'm not sure whether it really fits here either. What do you think @dlaz ?

[dlaz]

I agree with @130s's initial PR that this belongs somewhere in the PR2 repos, though I don't see an existing package that looks like a good fit. Another option which could be compelling is to add a people_simulation package which has this launch file, and whatever launch file was used to launch gazebo in the top screenshot. My only concern with that is that I'd rather not add gazebo as a dependency to people.

@DLu, a launch file for running the leg detector in simulation would be awesome too.

[130s]

Ok, I created people_app package; the launch files I'm willing to add aren't simulation specific, so I think app or sample might be more appropriate. How does this sound?
I'll add tests too.