Show only unread when selecting unread

Before, when there was no unread articles, all articles where displayed. This behaviour was not the one intended.
Now, when there is no unread articles, a message is displayed to alert that there is no article.

See #428
Show only unread when selecting unread

Before, when there was no unread articles, all articles where displayed. This behaviour was not the one intended.
Now, when there is no unread articles, a message is displayed to alert that there is no article.

See #428

Correction after comment
Merge branch 'issue428' of https://github.com/aledeg/FreshRSS into issue428

Conflicts:
	app/Controllers/indexController.php
Merge pull request #434 from aledeg/issue428

Show only unread when selecting unread