workaround webdav umlaut problems with mac os mavericks

Mac OS Mavericks can mount and browse Webdav-Directories without problems, even if the path contains umlauts. 
But you're not able to open any file within a path that contains any umlaut. 

Situation (afaik):
* Mavericks can browse Webdav with NFC-notated UTF-8
* Mavericks is unable to open (```open /Volume/<dav-mount>/<path>```) any file where the path contains a umlaut.

Solution: transform every path to UTF-8 NFD-notation.