Add 'anonymous' option to comment form

Add a field to the comment form to specify that the comment should be anonymous.

This is not enforced at the data storage level, the username is still recorded.  The consumer of the comment data (the front end) will have to check if is_anonymous is true and, if so, withhold the user's name accordingly.  For example:

```
<span>{% if node.is_anonymous %}Anonymous{% else %}{{node.user_name}}{% endif %}on {{node.submit_date|date:"M d, Y g:i A"}}</span>
```