Python: app module callable name configuration.

Now it is possible to specify the name of the application callable using
optional parameter 'callable'.  Default value is 'application'.

This closes #290 issue on GitHub.

src/nxt_main_process.c

@@ -191,6 +191,12 @@ static nxt_conf_map_t  nxt_python_app_conf[] = {
         NXT_CONF_MAP_STR,
         offsetof(nxt_common_app_conf_t, u.python.module),
     },
+
+    {
+        nxt_string("callable"),
+        NXT_CONF_MAP_CSTRZ,
+        offsetof(nxt_common_app_conf_t, u.python.callable),
+    },
 };