#281 no support for gif in graphviz on fedora 9

[Shay Halsband]

graphviz images generation failed.after some investigations I found that graphviz in my fedora 9 no longer support gif file type. I would recommend using png instead and maybe allowing a conf parameter with the image output type.

[Shay Halsband]

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BTW
replacing all instances of gif with png, in the code, fixed the problem for me.