Added new field: Heading

Hi, I have added a new field type named "heading". It'll be used as a captioned seperator for the fields on an options page. Here's a screenshot:

![scrshot](https://f.cloud.github.com/assets/3295/2076853/6db23c0a-8da9-11e3-8c51-371cd12c1a80.png)

You can define it in your config.php file like this:

```
array(
    'id'   =>'heading_1',
    'title' => __('Slide List Options', 'redux-framework-demo'),
    'type' => 'heading'
), 
```