Make docutils requirement explicit. Fixes #1243.

Previously, the error returned by Python when docutils is not installed
was not explicit, instead saying that HTMLTranslator is not defined
(needed by FeedGenerator and such), forcing the user to go into
readers.py to figure out that this happens because "import docutils"
failed.

This pull request makes the docutils dependency explicit, so that there
is an ImportError if doctutils is not found.