Change FetchFeed consturctor to use named parameters

This way you can initialize the FetchFeed class with passing a custom
logger, but without having to pass a parser.
Add integration test for feed importing

Covers:
- Initial import
- Update where no new entries are available
- Update where new entries are available
Add integration test for feed with invalid pub dates
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)
Ignore stories older than 3 days when importing feeds

Timestamps in feeds can not be trusted but we not some form of
preventing old articles be imported when a new feed is added or when
this branch is merged and the way Stringer handles story updates
changes. See discussion in
https://github.com/swanson/stringer/issues/261 and
https://github.com/swanson/stringer/pull/328.
Merge pull request #328 from pascalw/feature-improve-feed-importing-robustness

Improve feed importing robustness