Fix zdf_mediathek TypeError (#156)
Fix zdf_mediathek TypeError (#156)

* Fix zdf_mediathek TypeError

  File "C:\Python35\lib\site-packages\streamlink-0.0.2-py3.5.egg\streamlink\plugins\zdf_mediathek.py", line 95, in _get_streams
    formatList = res["priorityList"]["formitaeten"]
TypeError: list indices must be integers or slices, not str

* refactored zdf mediathek a bit, fixed bug with wrongly accessing priorityList

javiercantero/streamlink,streamlink/streamlink,chhe/streamlink,mmetak/streamlink,back-to/streamlink,wlerin/streamlink,gravyboat/streamlink,sbstp/streamlink,wlerin/streamlink,fishscene/streamlink,bastimeyer/streamlink,streamlink/streamlink,fishscene/streamlink,melmorabity/streamlink,beardypig/streamlink,beardypig/streamlink,sbstp/streamlink,ethanhlc/streamlink,chhe/streamlink,back-to/streamlink,mmetak/streamlink,melmorabity/streamlink,ethanhlc/streamlink,gravyboat/streamlink,javiercantero/streamlink,bastimeyer/streamlink