Add support for meta diffs

Related issue - #588

I've added two more arrays that contain the old and new values for all fields saved from within the metabox. 
This is especially useful if you need to create a diff between the two states of the post meta - for instance when you have to send an email with a summary of the changes.

@jtsternberg - looking forward to any comments/suggestions.
You can short-hand this by moving this to line 720:
```php
$old = self::$old_values[ $field->id() ] = $field->get_data();
```