Another attempt at fixing combinations of em and links

As requested by Julian Fagir (gnrp) in the [irc](http://irc.dokuwiki.org/index.php?d=2014-03-08#msg502297) channel I'm proposing this fix for the italics/em syntax.

This fixes cases with "normal" URLs but creates some weird special cases:
* ``this triggers // italics [[http://ärmel.de]] some test`` (``this does not trigger // italics [[http://www.ärmel.de]] some test``)
* ``//no italics://this is neither a url nor italics`` but ``//italics://this triggers italics because there are //more slashes later``