Fix notice

```
[09-Dec-2013 18:15:35 UTC] PHP Notice:  Undefined property: WritingHelper::$supported_post_types in /srv/www/vocativ.dev/content/plugins/writing-helper/writing-helper.php on line 53
```
Merge pull request #8 from Vocativ/fix-warning-notice

Fix notice: WritingHelper::$supported_post_types