Added a method to the User model that can be used to retrieve the entries marked as unread for a user that belong to a specific feed.
FeedsController#show method now only returns entries marked as unread for the current user.

This means that when clicking on a feed in the sidebar, only unread entries are shown.

In the future a button to "read all entries" will be added.
When retrieving unread entries in a feed from a User instance, the method makes sure that the user is actually subscribed to the feed from which entries are going to be retrieved. If the user is not subscribed to the feed, an error is raised.