Block old legacy clients

This Pull Request introduces a SabreDAV plugin that will block all older clients than 1.6.1 to connect and sync with the ownCloud instance.

This has multiple reasons:

1. Old ownCloud client versions before 1.6.0 are not properly working with sticky cookies for load balancers and thus generating sessions en masse
2. Old ownCloud client versions tend to be horrible buggy

In some cases we had in 80minutes about 10'000 sessions created by a single user. While this change set does not really "fix" the problem as 3rdparty legacy clients are affected as well, it is a good work-around and hopefully should force users to update their client