WIP: record the last time feeds in a folder were changed, with the new subscriptions_updated_at attribute of the Folder model.
When saving a Folder, the subscriptions_updated_at attribute is set to the current date/time unless another value has been already set.
WIP: Update the subscriptions_updated_at attribute of Folder instances at the appropiate times.
Declarative programming: when unsubscribing from a feed, update the subscriptions_updated_at attribute of the folder (if any) through ActiveRecord callbacks.
A bit of DRYing.

The work of updating the date/time of the last change in subscriptions is done in FeedSubscription#touch_subscriptions.

This method:

- touches the FeedSubscription instance, setting its updated_at attribute to the current datetime
- sets the subscriptions_updated_at attribute of the associated user to the current datetime
- if the feed is in a folder owned by the user, sets the subscriptions_updated_at attribute of the folder to the current datetime.

This method is invoked from multiple places:

- when creating a subscription
- when deleting a subscription
- when moving a feed into a folder
- when moving a feed out of a folder
- when the unread entries count of a feed changes
- when the title of a feed changes
- when the url of a feed changes
Use destroy instead of delete method to delete entries from the db during testing.

It currently doesn't affect test outcome, but in the future AR callbacks will probably be implemented in the Entry model, making destroy the right method to invoke.
WIP: update the subscriptions_updated_at attribute of folders when necessary.
Finished: tests to ensure that the subscriptions_updated_at attribute of Folder instances is updated when appropiate.
Implemented HTTP caching in the FeedsController#index action when listing feeds in a specific folder.

Also the case in which the folder passed as parameter in the request doesn't exist or is not owned by the user is better controlled (a log line is written where before just a 404 was returned).
When indexing feeds in a folder, do not pass a parameter with the current datetime in the Ajax request.

This was done in the past to keep angularjs from performing client-side caching, but it's no longer necessary and in facts prevents the client from using HTTP caching in this action.