Use django.conf.settings for project settings

To import project settings, you should use `django.conf.settings`, as it won't import correctly when it's installed to site-packages.

Error when trying to use middleware:

    >>> import error_pages.middleware.ErrorPageMiddleware
    Traceback (most recent call last):
      File "<console>", line 1, in <module>
      File "/usr/lib/python2.7/site-packages/error_pages/middleware.py", line 7, in <module>
        from error_pages import config
      File "/usr/lib/python2.7/site-packages/error_pages/config.py", line 1, in <module>
        import settings
    ImportError: No module named settings

[Reference](https://docs.djangoproject.com/en/dev/topics/settings/#using-settings-in-python-code).