Fix auto doc for modules using django import.

[pavlov99]

/home/vagrant/git/jsonapi/docs/jsonapi.rst:37: WARNING: autodoc: failed to import module u'jsonapi.resource'; the following exception was raised: Traceback (most recent call last): File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/sphinx/ext/autodoc.py", line 335, in import_object import(self.modname) File "/home/vagrant/git/jsonapi/jsonapi/resource.py", line 7, in from django.db import models File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/db/models/init.py", line 5, in from django.db.models.query import Q File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/db/models/query.py", line 17, in from django.db.models.deletion import Collector File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/db/models/deletion.py", line 4, in from django.db.models import signals, sql File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/db/models/sql/init.py", line 4, in from django.db.models.sql.subqueries import * File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/db/models/sql/subqueries.py", line 12, in from django.db.models.sql.query import Query File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/db/models/sql/query.py", line 22, in from django.db.models.sql import aggregates as base_aggregates_module File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/db/models/sql/aggregates.py", line 9, in ordinal_aggregate_field = IntegerField() File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/db/models/fields/init.py", line 116, in init self.db_tablespace = db_tablespace or settings.DEFAULT_INDEX_TABLESPACE File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/conf/init.py", line 54, in getattr self._setup(name) File "/home/vagrant/git/jsonapi/.env/lib/python2.7/site-packages/django/conf/init.py", line 47, in _setup % (desc, ENVIRONMENT_VARIABLE)) ImproperlyConfigured: Requested setting DEFAULT_INDEX_TABLESPACE, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.