OC users backend for the Chat app

For the Chat app we need to have the ownCloud users in the contacts app. I wrote a backend and it works better than expected :)

There are still a lot todo's (implement error checking) I will fix them asap. But can you have already a look at it?

@tanghus I have a few questions about the permissions. It shouldn't be possible to delete this address book. Neither it should be possible to delete or create contacts. But it should be possible to change the personal information about users. How can this be achieved? I have played with \OCP\PERMISSION_* but it didn't affected anything...

cc @jancborchardt