fix threaded comments html

This fixes issue #53 for me.

I changed this:
```
{% ifchanged comment.parent_id %}{% else %}
    </li>
{% endifchanged %}

{% if not comment.open and not comment.close %}
    </li>
{% endif %}
    
{% if comment.open %}
    <ul>
{% endif %}
```
to this:
```
{% if comment.open %}
    <ul>
{% else %}
    </li>
{% endif %}
```
I'm not sure what the reason was for the more complex logic before, but simplifying this has worked in every case I have tested so far.