feat(buttons): add controllerAs support

- Expose controller to the view for `btnCheckbox` and `btnRadio`
  directives

Closes #4140

src/buttons/buttons.js

@@ -14,6 +14,7 @@ angular.module('ui.bootstrap.buttons', [])
   return {
     require: ['btnRadio', 'ngModel'],
     controller: 'ButtonsController',
+    controllerAs: 'buttons',
     link: function (scope, element, attrs, ctrls) {
       var buttonsCtrl = ctrls[0], ngModelCtrl = ctrls[1];
 
@@ -45,6 +46,7 @@ angular.module('ui.bootstrap.buttons', [])
   return {
     require: ['btnCheckbox', 'ngModel'],
     controller: 'ButtonsController',
+    controllerAs: 'button',
     link: function (scope, element, attrs, ctrls) {
       var buttonsCtrl = ctrls[0], ngModelCtrl = ctrls[1];
 

src/buttons/test/buttons.spec.js

@@ -16,6 +16,17 @@ describe('buttons', function () {
       return el;
     };
 
+    it('should expose the controller to the view', inject(function ($templateCache) {
+      var btn = compileButton('<button ng-model="model" btn-checkbox>{{button.text}}</button>', $scope);
+      var ctrl = btn.controller('btnCheckbox');
+      expect(ctrl).toBeDefined();
+
+      ctrl.text = 'foo';
+      $scope.$digest();
+
+      expect(btn.html()).toBe('foo');
+    }));
+
     //model -> UI
     it('should work correctly with default model values', function () {
       $scope.model = false;
@@ -135,6 +146,17 @@ describe('buttons', function () {
       return el.find('button');
     };
 
+    it('should expose the controller to the view', inject(function ($templateCache) {
+      var btn = compileButtons('<button ng-model="model" btn-radio="1">{{buttons.text}}</button>', $scope);
+      var ctrl = btn.controller('btnRadio');
+      expect(ctrl).toBeDefined();
+
+      ctrl.text = 'foo';
+      $scope.$digest();
+
+      expect(btn.html()).toBe('foo');
+    }));
+
     //model -> UI
     it('should work correctly set active class based on model', function () {
       var btns = compileButtons('<button ng-model="model" btn-radio="1">click1</button><button ng-model="model" btn-radio="2">click2</button>', $scope);