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Fewer writes on feed refresh.

Instead of deleteing all stored articles and adding all articles
fetched from the feed, just store new articles and just delete
articles no longer sent from the source.

Fixes #54
Merge pull request #56 from eaburns/dev/eaburns

Fewer writes on feed refresh.
Fix datastore.Put.

I guess that I didn't test this in a case where new articles needed to be put.