Fix reports of unidentified users in Sym 2.6+

Sym 2.6.0 introduced a new getter for the author, so the old
Symphony::Engine()->Author was always null.

Funny to see that I needed this feature as well in the couple of last
commit, so we can already use Tracker::Author();

Re #37
Fixes #35
Fixes #34

Author: nitriques

extension.driver.php

@@ -549,11 +549,12 @@ public function parseLogin($context)
 
             // Set author ID. If author doesn't exist, store the IP
             // address.
-            if (Symphony::Engine()->Author) {
-                if (!$this->validateUser(Symphony::Engine()->Author->get('id'))) {
+            $author = Tracker::Author();
+            if ($author) {
+                if (!$this->validateUser($author->get('id'))) {
                     return;
                 }
-                $account = Symphony::Engine()->Author->get('id');
+                $account = $author->get('id');
             } else {
                 $account = 0;
                 $item = $_SERVER['REMOTE_ADDR'];