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_application.h

@@ -50,6 +50,7 @@ typedef struct {
     char       *home;
     nxt_str_t  path;
     nxt_str_t  module;
+    char       *callable;
 } nxt_python_app_conf_t;