Fix MySQL issues during fixture loading

I just installed django-threadedcomments into a project I'm working on but had some issues when I ran tests. In a nutshell, fixtures were failing to load because of foreign key restraint failures, e.g.:

```
IntegrityError: (1452, 'Cannot add or update a child row: a foreign key constraint fails (`test_myproject`.`threadedcomments_comment`, CONSTRAINT `comment_ptr_id_refs_id_fa2e61cf` FOREIGN KEY (`comment_ptr_id`) REFERENCES `django_comments` (`id`))')
```

I guess it's possible these fixtures load correctly on other backends? (I imagine this might be why it hasn't been caught until now.) But I had to make two changes to get the fixtures to load properly and for the tests in HierarchyTest to work:

* Invert the order of the comments and threaded comments in simple_tree.json.
* Set all values of last_child in simple_tree.json to null, and then update the values in a setUp() method of HierarchyTest.

The common thread in the issues is that if you attempt to add a row with an ID that references a row that has not yet been created, it fails. These fixes resolve this and tests run.

Anyhow, I'm submitting the changes I needed to make to get tests to run as a pull request. Hopefully you'll find this of use. Thanks