WebDAV depth infinity support - the dirty way

Contains ownCloud's own implementation of the Sabre_DAV_Server regarding the handling of depth infinity

The clean way would have been to submit proper patches to the upstream project and reincorporate the code once released.

This will and has to follow!

@guruz Thanks for the code in https://github.com/owncloud/3rdparty/pull/27/files 

@bartv2 @bantu @icewind1991 @karlitschek  review please
@dragotin @danimo @guruz @ogoffart please test if this is working as expected

litmus test results will follow ....