When successfully unsubscribing from a feed, the FeedsController returns an HTTP 204 code, instead of a 200. This is more in line with intended HTTP usage.

This will also be useful for the FoldersController: when removing a feed from a folder, a 204 will be returned if there are still feeds in the folder, and a 205 if there are no more feeds in the folder. This will tell the client whether to remove the folder from the view. This is not yet implemented.
Renamed the Folder.associate method to Folder.add_feed.

This is much more descriptive. Also this way it's more evident that the Folder.remove_feed I'm currently implementing is more-or-less the opposite to add_feed.
WIP: user can remove a feed from a folder.

The way it will be implemented, the user can:

- click on "no folder" in the dropdown, removing feed from all folders
- if the feed is in a folder, click on it in the dropdown to remove the feed from this folder

Currently both behaviors are exactly the same, because currently feeds can belong to a single folder for a single user. However, by implementing it this way, it would be easier to change the app to allow feeds to belong to several folders, if we so decide in the future.

Currently implemented: routing, FoldersController#destroy method, skeleton of the Folder#remove_feed method, FoldersController unit tests.

Not yet implemented: most of the Folder#remove_feed method, unit tests for this method, all javascript necessary for this to work client-side, acceptance tests.