Adding sorting functionality for conference folder view

With this patch it's possible to sort talks in conference folder
view by date, name, duration or random order. In addition a jump to
the top button was added on the button of the page.
Merge branch 'master' into sorting_conference_folder

Conflicts:
	layouts/browse-show-folder.haml
Do not include sorted by in breadcrumbs for normal pages
Talks length is sometimes nil

In our data base we have recordings with a missing recording length. As quick fix, we assume such talks witch a length of zero. In future we have to maintenance our data base.
Merge pull request #4 from voc/sorting_conference_folder

Adding sorting functionality for conference folder view