Change the latest duration to 24 hours.

Fixes #148.
Use the time an article was fetched instead of its publish time.

The publication times are unreliable. Often articles fail to show up
in the 'Latest' page, because when they are first found by feedme
their publish times are already too old. This fixes that by using the
time when feedme first fetch the article instead.

Fixes #147.
Merge pull request #149 from eaburns/dev/eaburns

Improvements for deciding on "latest" articles.
Store the article keys in the FeedInfo struct.

This allows us to avoid querying the datastore when checking for new
articles on refresh. Hopefully this will greatly reduce the number of
datastore "small operations" that we use.

Fixes #145.
Merge pull request #150 from eaburns/dev/articlekeys

Store the article keys in the FeedInfo struct.