Setting bgcolor attribute not possible

[flaushi]

Hi,
I am quite happy with the simple interface of this lib. However, when trying to change my outputs background color, I run into problems:

$g = new Graph();
        $v = $dc->buildGraph($g);
        $graphviz = new GraphViz();
        $graphviz->setFormat('svg');
        $g->setAttribute('graphviz.bgcolor', 'green');
        var_dump ($graphviz->createScript($g));

gives:

digraph G {   0 [label="/"]   1 [label="erloes21" color="red"]   2 [label="erloes2gesamt" color="red"]   0 -> 1   0 -> 2 }  [] []

and the output has white background.

[clue]

@flaushi Very good question! All graphviz graph attributes have to be added like this:

$graph = new Graph();
$graph->setAttribute('graphviz.graph.bgcolor', 'green');

I'll make sure this gets added to the readme once we tackle #2 +1