Assign labels as feeds are imported.

In case @rnalexander had a specific reason to use Beautiful Soup, this is a pull request instead of a commit. Merge if happy. I just don't like treading on people's toes. :)

--
Switches listparser for Beautiful Soup as this gives us the tags more
easily.

Also removes ZipFile parsing. Although it's convenient to just throw
a Google Reader takeout export at it, not everyone will be doing this
directly and it's no great hardship to unzip and find the XML yourself.

Fixes #13.