refactor(tabs): Change syntax, add features

* Tabs transclude to title elements instead of content elements, so the
ordering is always correct (Closes #153)
* Rename `<tabs>` to `<tabset>`, `<pane>` to `<tab>` (Closes #186)
* Add `<tab-heading>` directive, which is a child of a `<tab>`. Allows
HTML in tab headings (Closes #124)
* Add `select` attribute callback when tab is selected (Closes #141)
* Only the active tab's content is now actually ever in the DOM