Rename GvConvert to Image.

Update the GraphViz exporter so it's easy to override Gv::getNodeAttr…

…ibutes().

Add a new exporter GvConvert to export a Graphviz script into another…

… format.

Add more documentation for the Attribute node.

Issue #12: Consistency in constructor arguments.

Increase tests coverage.

Enable PHPSpec in GrumPHP.

Update badges look'n'feel.

Update badges look'n'feel.

Update composer.json.