Added control to avoid FeedClient entering an infinite loop when performing feed autodiscovery.

This could happen if :

- the class fetches an HTML document
- it tries to parse it as a feed and fails
- it performs feed autodiscovery on the HTML
- a feed is discovered, but its href points to the URL of the same HTML document
- the class fetches the HTML document again
- it tries to parse it as a feed and fails
- it performs feed autodiscovery on the HTML, then fetches the HTML doc again..
- infinite loop... no profit!

With the added control, when a feed is autodiscovered, it then is fetched but if FeedClient fails trying to parse it as a feed, it will NOT try to perform autodiscovery again. One try is enough.
Finished unit tests for the user subscription functionality.
Add acceptance test for subscribing to a feed by submitting the URL of its website (not of the feed itself).
User can subscribe to a feed submitting a URL without including the URI-scheme (for instance, "xkcd.com" instead of "http://xkcd.com". In this case, "http://" scheme is assumed.
Currently we don't support searching for feeds using search terms, only submitting URLs. Change i18n strings to reflect this.
Fixed a bug: when subscribing to a feed, the user was not automatically shown the entries of the feed.

As a side effect, this requires that the "Read all subscriptions" link under the "All subscriptions" folder is always present, even when the user has no subscriptions. This should not be a big problem.
When adding a subscription, immediately remove the entries of the feed the user was reading previously.
After subscribing to a feed, and while the entries of a feed are loading, the title of the entries list is hidden.