refactor(tabs): Change syntax, add features

* Tabs transclude to title elements instead of content elements, so the
ordering is always correct
* Renamed `<tabs>` to `<tabset>`, `<pane>` to `<tab>` (#186)
* Add `<tab-heading>` directive, which is a child of a `<tab>`. Allows
HTML in tab headings.
* Add `select` attribute callback when tab is selected (#141)
* Only the active tab's content is actually ever in the DOM