tab view component

[Raynos]

To demonstrate how to write a re-usable component using mercury we should author a few.

One that is interesting to author is a tab view component.

Basically a list of tab headers and a list of tab bodies, this will show how to handle internal state & DOM events. This component will also need a public interface to show any tab at any time.

We may also want to experiment with adding structural styling to the component using something like rcss.