XXE security issue.

Issue #414.
Merge branch '1.7' into 1.8

Conflicts:
	ChangeLog
1.8.9 release.
Missing note about sabre/vobject.
Merge branch '1.7' into 1.8
vobject note.
Merge branch '1.8'

Conflicts:
	ChangeLog
	lib/Sabre/DAV/Client.php