Bugfix: add missing `force-flat.html` to `setup.py`.

The exception is like this:

    Traceback (most recent call last):
      File "/home/vagrant/envs/plop-demo/local/lib/python2.7/site-packages/tornado/web.py", line 1021, in _execute
        getattr(self, self.request.method.lower())(*args, **kwargs)
      File "/home/vagrant/envs/plop-demo/lib/python2.7/site-packages/plop/viewer.py", line 35, in get
        data=profile_to_json(self.get_argument('filename')))
      File "/home/vagrant/envs/plop-demo/local/lib/python2.7/site-packages/tornado/web.py", line 498, in render
        html = self.render_string(template_name, **kwargs)
      File "/home/vagrant/envs/plop-demo/local/lib/python2.7/site-packages/tornado/web.py", line 602, in render_string
        t = loader.load(template_name)
      File "/home/vagrant/envs/plop-demo/local/lib/python2.7/site-packages/tornado/template.py", line 324, in load
        self.templates[name] = self._create_template(name)
      File "/home/vagrant/envs/plop-demo/local/lib/python2.7/site-packages/tornado/template.py", line 355, in _create_template
        f = open(path, "rb")
    IOError: [Errno 2] No such file or directory: '/home/vagrant/envs/plop-demo/lib/python2.7/site-packages/plop/templates/force-flat.html'