Get webcam working

This PR consists of two commits which together enable USB web cam support. The first commit adds an "apt-get" line to Docker file meaning that the USB webcam driver for ROS[0] gets installed into the container image. The second commit adds an example launch file which shows how to get video out of a USB web cam and display it in a window via ``image_view``[1].

[0] http://wiki.ros.org/usb_cam
[1] http://wiki.ros.org/image_view