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
In PR branch: panel created, new tab created

[1] What's the preferred way to refer to components? TabPanels, g-tabpanels, tabpanels, ...?