chore(buttons): add demonstration of multiple checkbox

- Add demonstration of how to store model when working with multiple checkboxes

Closes #3087
Closes #4065

src/buttons/docs/demo.html

@@ -5,7 +5,8 @@ <h4>Single toggle</h4>
         Single Toggle
     </button>
     <h4>Checkbox</h4>
-    <pre>{{checkModel}}</pre>
+    <pre>Model: {{checkModel}}</pre>
+    <pre>Results: {{checkResults}}</pre>
     <div class="btn-group">
         <label class="btn btn-primary" ng-model="checkModel.left" btn-checkbox>Left</label>
         <label class="btn btn-primary" ng-model="checkModel.middle" btn-checkbox>Middle</label>

src/buttons/docs/demo.js

@@ -8,4 +8,15 @@ angular.module('ui.bootstrap.demo').controller('ButtonsCtrl', function ($scope)
     middle: true,
     right: false
   };
+
+  $scope.checkResults = [];
+
+  $scope.$watchCollection('checkModel', function () {
+    $scope.checkResults = [];
+    angular.forEach($scope.checkModel, function (value, key) {
+      if (value) {
+        $scope.checkResults.push(key);
+      }
+    });
+  });
 });