Some little modifactions at examples.

Hi, I've put 4 spaces instead of tabs to fix Python coding guidelines. Also, I've added netevo prefix to some functions on examples.

At `visualization.py` I've tried to add `matplotlib.animation` to see the evolution of network as an animation on real time. I haven't finish it yet so you can ignore this change.

Also I've added `*.gml` and `*.png` to .gitignore because those are automatically generated.

There are some examples where I get a mistake. I'll report these as issues later.

Best regards.