If no Subject is passed into the Tracker constructor, use a default Subject

Author: fblundun

snowplow_tracker/tracker.py

@@ -83,6 +83,9 @@ def __init__(self, out_queue, _subject=None,
         if not log:
             logger.setLevel(logging.CRITICAL)
 
+        if _subject is None:
+            _subject = subject.Subject()
+
         self.out_queue = out_queue
         self.subject = _subject        
         self.encode_base64 = encode_base64