Default to Curl HttpClient.

+ AsyncHttpClient.configure("tornado.curl_httpclient.CurlAsyncHttpClient") is
  supposed to be used for this, but it runs into the same module naming
  confusion that tornadi_fix is used to resolve. Need to remove the
  tr/tornado symlink and sys.path.append("tr/vendor/tornado") instead.
diff --git a/tornado/httpclient.py b/tornado/httpclient.py
index 9f45b0a..b542939 100644
--- a/tornado/httpclient.py
+++ b/tornado/httpclient.py
@@ -116,8 +116,8 @@
         else:
             if cls is AsyncHTTPClient:
                 if cls._impl_class is None:
-                    from tornado.simple_httpclient import SimpleAsyncHTTPClient
-                    AsyncHTTPClient._impl_class = SimpleAsyncHTTPClient
+                    from tornado.curl_httpclient import CurlAsyncHTTPClient
+                    AsyncHTTPClient._impl_class = CurlAsyncHTTPClient
                 impl = cls._impl_class
             else:
                 impl = cls