Fix/reindex deletes settings

[PLNech]

Addresses #238 by ensuring we keep existing settings if any.

[PLNech]

Testing 831eeec locally succeeds everywhere but with django19:

  py27-django17: commands succeeded
  py33-django17: commands succeeded
  py34-django17: commands succeeded
  py27-django18: commands succeeded
  py33-django18: commands succeeded
  py34-django18: commands succeeded
  py35-django18: commands succeeded
  py36-django18: commands succeeded
  py27-django19: commands succeeded
ERROR:   py34-django19: commands failed
ERROR:   py35-django19: commands failed
ERROR:   py36-django19: commands failed
  coverage: commands succeeded

On django19, test_index_model_with_foreign_key_reference fails with this error:

Traceback (most recent call last):
  File "/Users/pln/Work/APIClients/algoliasearch-django/tests/test_index.py", line 65, in test_index_model_with_foreign_key_reference
    index.reindex_all()
  File "/Users/pln/Work/APIClients/algoliasearch-django/algoliasearch_django/models.py", line 424, in reindex_all
    if "Index does not exist" in e.message:
AttributeError: 'AlgoliaException' object has no attribute 'message'

Currently Investigating this error.