Support for django.contrib.sites 

This commit adds support for Django's site framework. The following is changed by this commit:

- a field `sites` is added to both models `Gallery` and `Photo`.  
- migrations are in place:
  - schema migration for the aforementioned fields
  - data migration to add the current site `settings.SITE_ID` to all existing objects.
- documentation for the feature and the new setting `PHOTOLOGUE_ADD_DEFAULT_SITE` is added as well
- tests are provided
- the changelog is updated and (and as a small extra) integrated into Sphinx docs)

This pull request should not be merged into master yet; as part of a Django behaviour described in [Ticket 14649](https://code.djangoproject.com/ticket/14649) automatically adding the default site does not work in Django's admin and must be done manually. I will provide another update to this branch once I have this fixed.