Mock servers shouldn't pollute test output.

BY writing to stderr, BaseHTTPRequestHandler writes log messages to the
console during testing.  This makes the output harder to interpret.
Write the log messages to stdout instead, so that test runners will
suppress them during passing tests, and show them during failing tests.

It would be nice to have a place to write this method just once for the
Youtube and LTI mock servers, but we don't seem to have a place for code
as common as that.

common/lib/xmodule/xmodule/util/mock_youtube_server/mock_youtube_server.py

 from BaseHTTPServer import HTTPServer, BaseHTTPRequestHandler
-import urlparse
+import json
 import mock
+import sys
 import threading
-import json
+import time
+import urlparse
 from logging import getLogger
+
+
 logger = getLogger(__name__)
-import time
 
 class MockYoutubeRequestHandler(BaseHTTPRequestHandler):
     '''
@@ -14,6 +17,15 @@ class MockYoutubeRequestHandler(BaseHTTPRequestHandler):
 
     protocol = "HTTP/1.0"
 
+    def log_message(self, format, *args):
+        """Log an arbitrary message."""
+        # Code copied from BaseHTTPServer.py. Changed to write to sys.stdout
+        # so that messages won't pollute test output.
+        sys.stdout.write("%s - - [%s] %s\n" %
+                         (self.client_address[0],
+                          self.log_date_time_string(),
+                          format % args))
+
     def do_HEAD(self):
         code = 200
         if 'test_transcripts_youtube' in self.path: