Coerce return value of 'selectedValueForPanel' to String

If 'selectedValueForPanel' is used to set the name of a new tab in g-tabpanels.html, the 'capitalize' function tries to call 'slice' on a Number. Since 'getAttribute' always returns a String, this ensures the method always returns a String.
Call 'initPanel' to add tabs for new panels

TabPanels implements 'initPanel', but Panels was not calling it after a panel was added.

'panelAdded' could take an index as well, but the tabpanels.html example always appends panels right now.
Merge pull request #1 from ssorallen/append-tabs-for-new-panels

Append tabs for new panels