Return new cache value

In ``Story.get_related_list`` return the new value after setting it in the
cache.  Before, there was a bug where it would only return a value if it
was found in the cache, but not if it had to first set the value in the
cache.

Addresses #566
Merge branch 'hotfix-566-lost_metadata' into develop

Conflicts:
	README.rst
	apps/storybase_story/tests.py