Use published scope by default

Since we have a property named `published_at` it makes sense to exclude
items that are not published from the feed.
Enable unpublishing of feed items
Move the use of published scope to the custom_scopes method
Use feedable’s published_at and unpublished_at if exist
Merge pull request #25 from hyperoslo/feature/only-show-item-if-published

Allow feed items to only be published in a certain period