Remove unnecessary require statement.
When there is a change in subscribed feeds, the subscriptions_updated_at attribute of the User instance is updated with the current date/time.
Added conditional GET support (HTTP caching) to FeedsController#index action, but only when listing all subscribed feeds.

HTTP caching when listing all feeds in a folder is not yet implemented.

HTTP caching uses only the last-modified header (no etag), and it's based on the subscriptions_updated_at attribute of the current user.
Do not pass the current time when requesting all feeds from the angularjs client.

Passing the current time as a parameter prevents the client from using HTTP caching properly.
User#subscribe method returns the subscribed feed.

This was recently changed and broke some things. Changed it so that the subscribed feed is returned again.