Add the class attribute to ion-tab directive

For this use case : http://forum.ionicframework.com/t/tabs-how-to-define-a-start-tab-which-has-no-icon/4165/4 and to be able to customize each tabs links.

For example, a default tab without its own link in the tabs-bar would be : 
```
<ion-tabs class="tabs-icon-top tabs-default tabs-positive">

  <!-- Welcome screen -->
  <ion-tab href="#/tab/welcome" class="hide">
    <ion-nav-view name="welcome-tab"></ion-nav-view>
  </ion-tab>

  <ion-tab title="Documents" icon="icon ion-document-text" href="#/tab/documents">
    <ion-nav-view name="documents-tab"></ion-nav-view>
  </ion-tab>

</ion-tabs>
```