Dockerfile: install USB webcam support for ROS
new launch file: usb_cam_test

Create a new launch file which shows an example of creating a node to
read video from a USB web cam and show it via a image_view node.
Makefile: expose /dev/video0 to the container

It turns out that unless ``-v /dev/video0:/dev/video0`` is added to the
Docker command line, the USB web cam device ends up with incorrect
permissions on some machines.
Merge pull request #4 from rjw57/get-webcam-working

Get webcam working