Allow anchors in wysiwyg.

#3859 

In order to allow the id attribute in elements for creating anchors we need to go to ```admin/config/content/formats/filtered_html```

and in "Rules for Element IDs" fill in

``` os-* ```

as the wysiwyg forces us to use a prefix for the id. 

![filtered_html](https://f.cloud.github.com/assets/4497748/1324500/10b14cde-34b3-11e3-840e-08e4b5610256.png)