use pre-link function in tabset instead of post-link,

Now controller's attributes $scope and $transcludeFn used in tabsetTitles post-link are surely initialized.

I had issue similar to https://github.com/angular-ui/bootstrap/issues/783 using tabs inside of ng-if directive - tabsetTitles postlink function ran before tabset link function, so i moved tabset link to pre-link and with angular-1.2.0-rc.3 it worked.

plunker with problem:
http://plnkr.co/edit/MoRweJ?p=preview

plunker with fix:
http://plnkr.co/edit/D3n45R?p=preview