feat(tabs): Support nested tabs

post-linking functions run on child directives may run first before
parents. In the case of nested directives, this was almost certain due
to the asynchronous nature of the linking (as they can only run after
the template has been loaded).

As such, the tabsetTitles expect the $transcludeFn to be available in
the TabsetController, it may not be available yet.

Unfortunately, using pre-linking should solve this but that's currently
broken in AngularJS 1.0.8 and was only fixed in 1.2.0
Ref: https://github.com/angular/angular.js/commit/31f190d4d53921d32253ba80d9ebe57d6c1de82b

Closes #783