Before this commit, t-raw a string containing `<svg>` did not display
the svg element. This was because of missing namespace in vnode's data.
Now, `vdom.addNS` is called to add the svg namespace when necessary.
If we want to generate dynamically svg components, it is useful to be
able to define components with a <g> tag as root, and then getting the
correct namespace.
closes#302
Michael Mattiello (mcm)