feat(tagsInput): Add ng-required support

Add support for the ng-required directive so the required validation key
is set when there are no tags in the model bound to the tagsInput directive.

Closes #157

src/tags-input.js

@@ -229,6 +229,10 @@ tagsInput.directive('tagsInput', function($timeout, $document, $window, tagsInpu
                 ngModelCtrl.$setValidity('leftoverText', scope.hasFocus || options.allowLeftoverText ? true : !scope.newTag.text);
             };
 
+            ngModelCtrl.$isEmpty = function(value) {
+                return !value || !value.length;
+            };
+
             scope.newTag = {
                 text: '',
                 invalid: null,

test/tags-input.spec.js

@@ -1676,6 +1676,36 @@ describe('tags-input directive', function() {
         });
     });
 
+    describe('ng-required support', function() {
+        it('sets the required validation key when there is no tags', function() {
+            // Arrange/Act
+            compileWithForm('name="tags"', 'ng-required="true"');
+
+            // Assert
+            expect($scope.form.tags.$invalid).toBe(true);
+            expect($scope.form.tags.$error.required).toBe(true);
+        });
+
+        it('doesn\'t set the required validation key when there is any tags', function() {
+            // Arrange/Act
+            $scope.tags = ['Tag'];
+            compileWithForm('name="tags"', 'ng-required="true"');
+
+            // Assert
+            expect($scope.form.tags.$invalid).toBe(false);
+            expect($scope.form.tags.$error.required).toBe(false);
+        });
+
+        it('doesn\'t set the required validation key when ng-required is false', function() {
+            // Arrange/Act
+            compileWithForm('name="tags"', 'ng-required="false"');
+
+            // Assert
+            expect($scope.form.tags.$invalid).toBe(false);
+            expect($scope.form.tags.$error.required).toBe(false);
+        });
+    });
+
     describe('autocomplete registration', function() {
         var autocompleteObj;