demo(buttons): use labels instead of buttons for groups

 * Align with BS3 markup.

Closes #1323. Closes #1482

Author: bekos

src/buttons/docs/demo.html

@@ -7,15 +7,15 @@ <h4>Single toggle</h4>
     <h4>Checkbox</h4>
     <pre>{{checkModel}}</pre>
     <div class="btn-group">
-        <button type="button" class="btn btn-primary" ng-model="checkModel.left" btn-checkbox>Left</button>
-        <button type="button" class="btn btn-primary" ng-model="checkModel.middle" btn-checkbox>Middle</button>
-        <button type="button" class="btn btn-primary" ng-model="checkModel.right" btn-checkbox>Right</button>
+        <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>
+        <label class="btn btn-primary" ng-model="checkModel.right" btn-checkbox>Right</label>
     </div>
     <h4>Radio</h4>
     <pre>{{radioModel}}</pre>
     <div class="btn-group">
-        <button type="button" class="btn btn-primary" ng-model="radioModel" btn-radio="'Left'">Left</button>
-        <button type="button" class="btn btn-primary" ng-model="radioModel" btn-radio="'Middle'">Middle</button>
-        <button type="button" class="btn btn-primary" ng-model="radioModel" btn-radio="'Right'">Right</button>
+        <label class="btn btn-primary" ng-model="radioModel" btn-radio="'Left'">Left</label>
+        <label class="btn btn-primary" ng-model="radioModel" btn-radio="'Middle'">Middle</label>
+        <label class="btn btn-primary" ng-model="radioModel" btn-radio="'Right'">Right</label>
     </div>
 </div>