The Refresh button is initially hidden, shown when clicking on a feed. From that point on, each time a feed is clicked on the Refresh button is disabled while the feed is loading.

Added non-functional and disabled "Unsubscribe" button.
Unsubscribe button is hidden until a feed is clicked on. From that moment on, each time a feed is clicked on the Unsubscribe button is disabled while the feed loads.
Removed rdoc comment in FeedsController#create that referenced a currently unimplemented functionality (search for feeds matching a search string).
User can unsubscribe from a feed by clicking on the Unsubscribe button and accepting the confirmation popup.

Some acceptance tests are currently failing because of a change in the generated HTML (the <li> for each feed entry in the sidebar now does not have an id generated from the feed id, because it lead to repeated ids if the feed was in a folder; instead data-* attributes are used). The failing tests relied on the old HTML ids to check for the presence of some elements in the page; next commit will fix them.

Some things are still unimplemented: showing alerts if there's a problem, disabling the Unsubscribing button when reading a whole folder, deleting the feed if no users are subscribed to it anymore, showing the main "welcome&statistics" page after deleting.

This last one is interesting. Currently the welcome&statistics view is unimplemented. We're currently making the user read the whole list of entries for all feeds after unsubscribing from a feed, but this is not optimal because this list of entries is the longest one to load. It could get annoying for the user!
Fixed broken acceptance tests, to reflect latest changes in generated HTML for the sidebar.

Also fixed the fragment that generates the <li> for each feed in the sidebar. Now the data-feed-id attribute of the link has a value of the id of the feed, instead of the "feed-#{id}" value it had before.
Show an alert if there's a problem unsubscribing from a feed.
After unsubscribing a user from a feed, if there are no more users subscribed, totally delete the feed from the database.

This in turn launches a delete of all its entries.
Unsubscribe button is hidden and disabled while reading a whole folder (a "Read all subscriptions" link is selected in the sidebar). It's only shown and enabled while reading a single feed.

Ensure that clicking on Refresh and Unsubscribe buttons does nothing if the button is disabled.
Remove completely a feed from the sidebar after unsubscribing.
By default a start page (still unimplemented) is shown. When clicking on a feed the start page is hidden. Added a "Start" link on top of the sidebar; when clicking on this feeds the start page is shown again.
Added acceptance test to ensure that if several users are subscribed to a feed, when one of them unsubscribes the rest are not affected.
When subscribing to a feed, while the list of entries loads the start page will be hidden. If there is a problem subscribing to the feed, the start page will be shown (regardless where the user was when he clicked on the "Add subscription" button).