Add transparent background

[witrin]

Remove the background polygon from Graphviz and replaces rsvg-convert with convert.

[witrin]

The replacement with Imagemagick's convert was just a quick move because rsvg-convert doesn't have rendered the background transparent, but maybe this was a installation problem on my server (2.40.13).

[witrin]

@aivarsk Would you be so kind a leave a comment?

[aivarsk]

Well, I don't spend much time thinking about this as I use plantuml for my own UML needs.
Adding transparent background is ok but I don't like/understand why you removed the main function from scruffy.py. It puzzles me as one of use cases for me was running scruffy.py on different svg files.
So if you make a pull request the background only I would pull it. And the removal of main() is separate topic/pull request that has to be discussed.

[witrin]

Alright, thanks! :)