Limit number of articles added to the datastore.

Otherwise, feeds can serve hundreds of articles which would really be
hard on our quota. Other readers (like poor old Google reader) limit
their initial fetching to about 10 articles too.

Fixes #60
Use consts for datastore entity kinds.

Fixes #58
Merge pull request #61 from eaburns/dev/eaburns

A couple of small fixes