Add support for namespaced user model

Author: freekmurze

src/Spatie/Activitylog/Models/Activity.php

@@ -2,6 +2,7 @@
 
 use Eloquent;
 use DateTime;
+use Config;
 
 class Activity extends Eloquent {
 
@@ -19,7 +20,7 @@ class Activity extends Eloquent {
      */
     public function user()
     {
-        return $this->belongsTo('User', 'user_id');
+        return $this->belongsTo(Config::get('auth.model'), 'user_id');
     }
 
     protected $guarded = array('id');
@@ -34,4 +35,4 @@ public static function boot()
             $activity->created_at = new DateTime();
         });
     }
-}
+}