fix(tabs): make tab links keyboard accessible

Fixes angular-ui#2226
Fixes angular-ui#2290
Closes angular-ui#2870
Closes angular-ui#2304

Author: blah238

template/tabs/tab.html

@@ -1,3 +1,3 @@
 <li ng-class="{active: active, disabled: disabled}">
-  <a ng-click="select()" tab-heading-transclude>{{heading}}</a>
+  <a href ng-click="select()" tab-heading-transclude>{{heading}}</a>
 </li>