add ZDFIE

Hello,

i've added a IE for zdf.de/ZDFmediathek.
It's choose the best quality for Windows Media Player links. The Quicktime links doens't work, so it seems.
'mmsclient' doesn't download these mms:// link anymore, so i choose 'mplayer'. i hope this in ok. Alternative downloader/ripper are welcome.

My test enviroment:
Python 3.3.1
MPlayer SVN-r29137-4.3.2 (C) 2000-2009 MPlayer Team

successful tested videos:
http://www.zdf.de/ZDFmediathek/beitrag/video/1895898/Vorschau-zur-Sendung-vom-28.05.2013?bc=sts;suc
http://www.zdf.de/ZDFmediathek/beitrag/video/587828/So-wird-das-Wetter?bc=sts;sta