fix logger implementation

I was trying to use the Logger implementation returned by `\OC::$server->getLogger()`. It dies horribly whenever trying to log anything because it instantiates the `$logger` member as a string `'OC_Log_Owncloud'` in init() and then tries to call a method on it with `$this->logger::write(...)`. I assume the idea of the current constructor is to allow passing in an `ILogger` instance or maybe even a `Psr\Log\LoggerInterface` which is why I made log() handle assume $logger to be an object first and only then make a `call_user_function`