Replacing PHPass with password_compat

Per https://github.com/owncloud/core/issues/10050#issuecomment-51333177

also https://github.com/owncloud/3rdparty/pull/110
> It needs to be there in the off case we ever change the hashing algorithm in the future. or if the user adjusts the hashing cost

That contradicts 

> if the password is using the old method it will always need rehashing