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
This causes a fatal error in my local install, I activated this extension but did not configure it, no problem but after cherry-picking this pull request, I cannot log anymore since updateInterval has not been configured.

As such I would suggest to NOT throw the exception but return a boolean FALSE and checking it on line 94 to skip the synchronization.

SAME for the v4 version