Add support group element. #11

I think we can simulate the "g" element. In append if we treat "g" as a special case, we could return a special D3 group selection which is basically a Raphael set. If a transformation is applied to the group, it will apply it to every element in the set. Likewise, it will store the transformation string and apply it to all elements that are "appended" to it. This should be a pretty close simulation to actual SVG group elements except they can't be "selected" and directly apply all attributes to the elements in the set (unless we limit it to transformations).

Let me know your thoughts on this and I can probably start working on it.