use scope for public subscriptions
use scope for stored_since items

It's safer not to make relation from the lower to the upper in factory.
use scope for crawlable feeds
use scope for has_unread subscription
extract Feed#avg_rate method
use scope for recent subscriptions
use scope for past pins
use scope for recent items
use `includes` instead of `find(:all)`
use scoped instead of `find(:all)`
use scoped.first instead of `find(:first)`
allow without argument call to `Item.recent`
Merge pull request #106 from onk/improve_activerecord_deprecated_finders

Improve activerecord deprecated finders