Allow .node>circle to receive css styles

[bfriedz]

Updates the utils.js and utils.spec.js files to support styles for .node>circle.
Addresses issue #443

Using this code in circleColorByClassDef.html

    graph LR;
    A((start))-->B(step1);
    B-->C[step2];
    C-->D{step3};
    D-->D2(-step3.5-);
    D2-->E[end];
    classDef yellow fill:#ffff00,stroke:#333,stroke-width:3px;
    class A,B,C,D,D2,E yellow;

[knsv]

Great! Thanks.