Support for linking slides to url added

Probably not the best way, but it works. A custom field named url will
have to be added manually in order to add a link to a slide. Should not
affect existing installations in any other way.
Reduntant url escaping, one should be enough.
Changed to reflect new functionality (url linking).
Merge pull request #2 from atnon/master

Added linking slide image to url. Great stuff, thanks @atnon!