Allow parent class to implement new logging method

Sometimes, applications require a different logging method than the standard CSV output, for instance using Monolog to send logs to a variety of destinations, including emails or storing in a database. By making this method protected instead of private, an application can extend iSDK and implement its own log function. At the moment, a parent class must also implement its own methodCaller function to achieve this.

For instance, this change allows this code to work easily:

```
class iSDKTracker extends iSDK {
  protected function log($data) {
    var_dump($data);
  }
}
```