feat(popover): use expression to fix usage with $sce

Allows for trusted resource URLs through Strict Contextual Escaping ($sce).
If the an interpolated expression is used instead, then the benefits of SCE is
lost.

Fixes #3558

src/popover/docs/demo.html

@@ -14,7 +14,7 @@ <h4>Dynamic</h4>
     </div>
     <button popover="{{dynamicPopover.content}}" popover-title="{{dynamicPopover.title}}" class="btn btn-default">Dynamic Popover</button>
 
-    <button popover-template="{{dynamicPopover.templateUrl}}" popover-title="{{dynamicPopover.title}}" class="btn btn-default">Popover With Template</button>
+    <button popover-template="dynamicPopover.templateUrl" popover-title="{{dynamicPopover.title}}" class="btn btn-default">Popover With Template</button>
 
     <script type="text/ng-template" id="myPopoverTemplate.html">
         <div>{{dynamicPopover.content}}</div>

src/popover/popover.js

@@ -9,7 +9,7 @@ angular.module( 'ui.bootstrap.popover', [ 'ui.bootstrap.tooltip' ] )
   return {
     restrict: 'EA',
     replace: true,
-    scope: { title: '@', content: '@', placement: '@', animation: '&', isOpen: '&',
+    scope: { title: '@', contentExp: '&', placement: '@', animation: '&', isOpen: '&',
       originScope: '&' },
     templateUrl: 'template/popover/popover-template.html'
   };

template/popover/popover-template.html

@@ -7,7 +7,7 @@
   <div class="popover-inner">
       <h3 class="popover-title" ng-bind="title" ng-if="title"></h3>
       <div class="popover-content"
-        tooltip-template-transclude="content"
+        tooltip-template-transclude="contentExp()"
         tooltip-template-transclude-scope="originScope()"></div>
   </div>
 </div>