Moved the code that updates a feed to a new class lib/feed_updater. UpdateFeedJob just invokes this class.

This enables us to create a different job for when the update is manually requested by a user. This job will receive as an additional argument the id of the user, will also invoke lib/feed_updater and additionally will update the status of the job in the database, so that the user can see this status in the view.
Renamed RefreshFeedJob model to RefreshFeedJobStatus.

The convention is that classes with names ending in "Job" are Resque job classes. It's a bad idea having an ActiveRecord model named like that. Hopefully the new name is less confusing. I know it's a verbose class name. I've been programming enterprise java for years before coming to rails, I may have issues. Leave me alone or send me a pull request with better naming.