Repository handles subscribe to url

Instead of create a feed then create a subscription in the higher
application, the repository accepts subscriptions for a url and
creates the feed if it is necessary.
Refactor: Make Repository interface methods public
Rename CopyFeedsAsJSONBySubscribedUserID
Fix CopySubscriptionsForUserAsJSON when none

fixes #3
Remove feed when last user unsubscribes

fixes #1
v0.1.5