Defer import of django.conf.settings

This fixes installation issues related to this file being imported into setup.py

Without this change I intermittently recieve the following error:

```
Downloading/unpacking django-simple-menu==1.0.6 (from -r requirements.txt (line 52))
  Running setup.py egg_info for package django-simple-menu
    Traceback (most recent call last):
      File "<string>", line 14, in <module>
      File "/Users/jacob/.virtualenvs/gizmag/build/django-simple-menu/setup.py", line 5, in <module>
        from menu import VERSION_STRING
      File "menu/__init__.py", line 1, in <module>
        from django.conf import settings
    ImportError: No module named django.conf
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):

  File "<string>", line 14, in <module>

  File "/Users/jacob/.virtualenvs/gizmag/build/django-simple-menu/setup.py", line 5, in <module>

    from menu import VERSION_STRING

  File "menu/__init__.py", line 1, in <module>

    from django.conf import settings

ImportError: No module named django.conf
```