Append tabs for new panels

Very cool work, Polymer. The g-tabpanels[1] component and associated tabpanels example support creating new panels, but they do not create associated tabs. g-tabpanels implemented `initPanel` as intended by g-panels, but g-panels was not calling it when `panelAdded` was called.

Try this on master and then in the branch:

1) Open the tabpanels.html example
2) Click the "+" button to create a new panel

In master: panel created, no new tab
![screen shot 2013-05-16 at 11 23 05 pm](https://f.cloud.github.com/assets/29612/516561/4fc226f6-beba-11e2-9b20-7050df012127.png)

In PR branch: panel created, new tab created
![screen shot 2013-05-16 at 11 21 28 pm](https://f.cloud.github.com/assets/29612/516562/5cd94798-beba-11e2-8aa0-dc639dfd11e3.png)

[1] What's the preferred way to refer to components? TabPanels, g-tabpanels, tabpanels, ...?