Basic fix for issue #31

This uses an LRUDict internally to ArticleCollection to decide which articles to drop when the collection gets full, rather than just dropping the articles with the lowest article_id.

I'm pretty sure this fixes the issue but it's a little tricky to write a test--I need to refactor the recsystem class a bit to make it easier to write simple regression/unit tests, as opposed to the more complex functional tests that already exist.