move the private namespace OC into lib/private - OCP will stay in lib/public

Conflicts:
	lib/private/vcategories.php
reorganize file in lib
Merge branch 'master' into lib-private-master-2
Merge branch 'master' into lib-private-master-2
Merge pull request #5039 from owncloud/lib-private-master-2

Public API OC6 - introduce lib/private