Creates fieldset functionality

Now you can make sets of fields within a meta box. Just pass a `fields` array value with `type = fieldset`, then inside a key called `fields` pass the form fields like normal. Like this:
```
'phone-2' => array(
                'title' => 'Alternate Phone number',
                'slug' => 'phone-2',
                'type' => 'fieldset',
                'fields' => array(
                    array(
                        'type' => 'text',
                        'max_length' => 11,
                        'label' => 'Number',
                    ),
                    array(
                        'type' => 'text',
                        'max_length' => 40,
                        'label' => 'Description',
                    ),
                ),
                'params' => array(),
                'max_length' => 10,
                'meta_key' => 'phone-2',
                'howto' => 'This can be an alternative contact number. Use the description field to explain what it is.',
            ),
```
Build status from Travis: https://travis-ci.org/gboone/cms-toolkit
Given its behavior, does it make sense to run this block immediately after setting `$key` and before doing anything else? The setting of `$existing` is not likely expensive; just wondering.