php-yubico: example files removed
Cleaner implementation + logging

- Moved otp and password checks into a new class, which can be
found in a new file called "auth.php".
- user_yubiauth: added logging for otp verification failures
auth: modified dependency + small update

Auth now uses "php-yubico" inside the app/3rdparty folder,
no need to move it to owncloud/3rdparty anymore.
ajaxify settings: some modifications + clean-up
ajaxify settings: javascript for handling ajax post requests
ajaxify settings: server-side php scripts with csrf protection

These scripts communicate with the client-side javascript functions.
The code in these files was previously (mostly) found in
"app/settings.php" and "app/settings_admin.php". The most important
addition to the older code is the CSRF protection.