fix(a11y): prevent list from being focusable

- Explicitly disallow focusing of list, mainly fixes behavior in Firefox

Closes angular-ui#898

Author: piebev

src/select2/choices.tpl.html

@@ -1,4 +1,4 @@
-<ul class="ui-select-choices ui-select-choices-content select2-results">
+<ul tabindex="-1" class="ui-select-choices ui-select-choices-content select2-results">
   <li class="ui-select-choices-group" ng-class="{'select2-result-with-children': $select.choiceGrouped($group) }">
     <div ng-show="$select.choiceGrouped($group)" class="ui-select-choices-group-label select2-result-label" ng-bind="$group.name"></div>
     <ul role="listbox"
@@ -8,4 +8,4 @@
       </li>
     </ul>
   </li>
-</ul>
+</ul>