asset: Add a manager to load and handle assets.

    - Add AssetManager class, which tries to load all asset files from a folder.
    - Added abstract load function to ARenderableObject.
    - Added load function to ASCIIPicture, which loads ASCII graphics from txt files.
    - Add ImagePicture class, which loads pixel graphics.
    - Add cat.txt ASCII image.

Signed-off-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
tools: Add Logger and Manager to handle them

    - Add Logger which writes messages to a file with time stamps
    - Add Manager that handle loggers, and provides a default logger.

Signed-off-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
tools: Replace prints to console

    - Prints to console now use the new logger

Signed-off-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>