Tested functions:
- nodelets
- one nodelet accessing different topics
- one nodelet listening to different topics
- using boost::threads in nodelets
- using .launch files
- using customized messages
- using folder structure for different nodelets

Added the .gitignore file, to prevent git from pushing build/ and devel/
from catkin-ws.
Added some information for using docker.

With the next commit there will be a .gitignore file to prevent git from
pushing folders like "catkin-ws/build" oder "catkin-ws/devel".