Introducing application 'atexit' hook.

Finalizing Python interpreter.

This closes #65 issue on GitHub.

src/nxt_go.c

@@ -20,6 +20,7 @@ nxt_application_module_t  nxt_go_module = {
     nxt_string("go"),
     nxt_go_init,
     nxt_go_run,
+    NULL,
 };