Backported deserializer for {DAV:}current-user-privilege-set property.

Issue #335
Merge branch '1.7' into 1.8

Conflicts:
	lib/Sabre/DAVACL/Version.php
Fixed deserializer for 1.8
Merge branch '1.8'

Conflicts:
	lib/Sabre/DAVACL/Property/CurrentUserPrivilegeSet.php
	lib/Sabre/DAVACL/Version.php