Implemented the Folder#remove_feed method. This methods removes a feed from a folder and, if no more feeds are in the folder, deletes it from the database.

In the end I've decided to remove support for passing the value "all" instead of a folder_id to the method. This would have meant passing the user id as an argument, because we never want to remove a feed from ALL folders, at most we could want to remove it from all folders belonging to a single user.

As this is a option that was not going to be used at the moment and which adds some unnecessary complexity, it's been removed from the roadmap for now.
Completed work in FoldersController and its unit tests, to support the #destroy method.
Removing a feed from a folder is done with the FoldersController#remove method. The FoldersController#destroy method is unused and removed.

The new method is similar to the old one, with the difference that the folder id is not passed as a param, but instead is inferred (any feed can belong to at most one folder for the current user; this is enforced by methods in the Folder and Feed models).

The route is also changed; instead of the RESTful DELETE /folders/:id route, the DELETE /folders/remove route is used. It is mandatory that the feed_id is present in the params.

This change is desirable because of the way client interaction works. The client sends an order to remove a given feed from all folders, and it's not necessary that the client specifies the folder under which the feed is currently.
Client-side support for removing a feed from its folder.

Some corner cases are not controlled yet.
DRY the coffeescript code related to folder management. Grouped shared code in separate functions.