[RFR] Add callback field type

Usage:
```js
// In the app
app.directive('customPostLink', ['$location', function($location) {
    return {
        restrict: 'E',
        template: '<a ng-click="displayPost(field)">View post</a>',
        link: function($scope, element, attributes) {
            $scope.displayPost = function(field) {
                var postId = 'post_id_raw' in field ? field.post_id_raw : field.entity.getField('post_id').value;

                $location.path('/edit/posts/' + postId);
            }
        }
    }
}]);

// In the configuration
Entity('comments')
.addField(Field('actions')
        .type('callback')
        .list(true)
        .label('Actions')
        .isEditLink(false)
        .callback(function() {
            return '<custom-post-link></custom-post-link>';
        })
)
```