Only refresh feeds when showing their articles.

Previously, we considered refreshing stale feeds every time the
FeedInfo was read from the datastore. This was unnecessary.

Also, when adding a feed, it was refreshed. But, if the refresh failed
then the feed was never subscribed, and it's reference count was never
incremented even though the FeedInfo was put in the datastore. Now, we
don't refresh the feed until we actually want it's articles, so this
issues goes away.

Fixes #63
Use methods.

This way we don't need to reread the FeedInfo from the datastore in
some cases, and it allows for clearer names (f.articles, not
getArticles, etc.).
Better error reporting when displaying feeds.

Show successful feeds, and simply list any errors encountered in a
separate "article" at the top of the page. This is much better than
the previous behavior or sending an HTTP internal server error if any
of the feeds had an error.

Fixes #36
Merge pull request #65 from eaburns/dev/reporterrors

Better error reporting when displaying feeds.