feat(tabs): Change directive name, add features

* Rename 'tabs' directive to 'tabset', and 'pane' directive to 'tab'.
 The new syntax is more intuitive; The word pane does not obviously
 represent a subset of a tab group. (Closes #186)
* Add 'tab-heading' directive, which is a child of a 'tab'. Allows
HTML in tab headings. (Closes #124)
* Add option for a 'select' attribute callback when a tab is selected.
 (Closes #141)
* Tabs transclude to title elements instead of content elements. Now the
 ordering of tab titles is always correct. (Closes #153)
* Only the active tab's content is ever present in the DOM. Provides an
 increase in performance.

BREAKING CHANGE: The 'tabs' directive has been renamed to 'tabset', and
 the 'pane' directive has been renamed to 'tab'.

    To migrate your code, follow the example below.

    Before:

    <tabs>
      <pane heading="one">
        First Content
      </pane>
      <pane ng-repeat="apple in basket" heading="{{apple.heading}}">
        {{apple.content}}
      </pane>
    </tabs>

    After:

    <tabset>
      <tab heading="one">
        First Content
      </tab>
      <tab ng-repeat="apple in basket" heading="{{apple.heading}}">
        {{apple.content}}
      </tab>
    </tabset>