Fixed RuntimeWarning by replacing naive datetime by utc
Added more tests and refactored the the first one.
Fixed on_article_delete if the article has children.

Reported by tests.py: "variable referenced before assignment"
Added tests for clearing cache and for updating article_list
Fixed on_article_delete_clear_cache. Some articles in the cache were not cleared and tests failed.
Merge pull request #141 from hynekcer/master

Added more tests and fixed bugs