Change feed importer to use story ids to determine if a story is new

This should make feed importing more robust. Pub dates of feeds and
articles can not always be trusted (see
https://github.com/swanson/stringer/issues/261).
Using this mechanism we should always reliably be able to tell which stories
are new, regardless of pub dates being correctly reported or not.
Adhere to project code style (string double quotes)