Added the possibility to synchronize the users from master repository in a specific interval

The synchronization happens before backend user login. The interval can be set in the configuration of the extension.

I'm a trainee at [Bitmotion](http://www.bitmotion.de) where we manage a lot of TYPO3 sites and wanted to be able to update all user credentials in a more easy way. We hope to get this patch upstream.

Best regards
Daniel
you hardcoded multiple times 'causal_accounts' (maybe I did it as well in my original code) but I use to like having a 

static protected $extKey = 'causal_account';

at the beginning and then using static::$extKey instead, nice to have but I won't block if not done