Support handler-encoding-ignorant embedders (#32)

Allow for embedders that don't provide the new handler_encoding option
inside the options object by using getattr() and falling back to None.
diff --git a/mod_pywebsocket/websocket_server.py b/mod_pywebsocket/websocket_server.py
index c9861bc..9f67c9f 100644
--- a/mod_pywebsocket/websocket_server.py
+++ b/mod_pywebsocket/websocket_server.py
@@ -85,11 +85,15 @@
         if necessary.
         """
 
+        # Fall back to None for embedders that don't know about the
+        # handler_encoding option.
+        handler_encoding = getattr(options, "handler_encoding", None)
+
         # Share a Dispatcher among request handlers to save time for
         # instantiation.  Dispatcher can be shared because it is thread-safe.
         options.dispatcher = dispatch.Dispatcher(
             options.websock_handlers, options.scan_dir,
-            options.allow_handlers_outside_root_dir, options.handler_encoding)
+            options.allow_handlers_outside_root_dir, handler_encoding)
         if options.websock_handlers_map_file:
             _alias_handlers(options.dispatcher,
                             options.websock_handlers_map_file)