Comments in `yolo_tiny_net.py`

[flonkler]

When I looked through this fascinating project as a machine learning and tensorflow noob, I tried to add comments for myself in order to understand what's going on. I'd like to share those comments, so that other folks can profit from them as well. Tell me if I should do it for yolo_net.py as well.

By the way, I also added a .gitignore file since it should be avoided that commits contain someone's training checkpoints or *.pyc files.

Have a nice day!