Hide group audience field in a collapsed fieldset

#5089 

#### Changes

* In `os_form_node_form_alter()`, move the group audience field into a fieldset.

#### Before

![screen shot 2014-02-07 at 2 02 19 pm](https://f.cloud.github.com/assets/228733/2113262/b8fb48b8-902a-11e3-84dd-450de096ebf6.png)

#### After

![screen shot 2014-02-07 at 2 03 05 pm](https://f.cloud.github.com/assets/228733/2113259/b5965a6e-902a-11e3-98a0-fde95ef0252f.png)