Add CHANGELOG to Sphinx docs.

And reformat CHANGELOG to 80 characters line width.
Add support for django.contrib.sites (fixes #84)
Refine automatic adding of Photos to the current site.

Only Photos are affected by PHOTOLOGUE_ADD_DEFAULT_SITE now, and only on
creation and when no sites are specified.

The current site is selected initially when creating a Gallery or Photo through
the admin; this selection can be changed by the user.
Fix tests for site support.

Extend factories for Gallery and Photo with `sites` kwarg.
Warn users if photos in gallery will not display.
Typo.
Docs: tell users to install the Sites framework.
Warn users if photos in gallery will not display.
Add field `sites` to ModelAdmin.list_filters.
Add site related admin actions for Gallery and Photo.
Make a start on chainable manager methods.
Merge remote-tracking branch 'Jnns/django.contrib.sites-support' into django.contrib.sites-support

Conflicts:
	photologue/admin.py
Galleries/Photos get associated with the current site via a manager query.
Trivial reformatting.
Changed slightly how multi-site support works.
Changelog tweaks.
Bug fix the 'previous/next photo in gallery' methods.