Correctly use the subparser widgets dictionary

When generating the GUI using subparsers, the subparser widgets dictionary is used instead of the parent one. (Issue #112)

gooey/python_bindings/argparse_to_json.py

@@ -45,7 +45,8 @@ def convert(parser):
     if has_required(actions):
       raise UnsupportedConfiguration("Gooey doesn't currently support required arguments when subparsers are present.")
     layout_type = 'column'
-    layout_data = {name.lower(): process(sub_parser, widget_dict) for name, sub_parser in get_subparser(actions).choices.iteritems()}
+    layout_data = {name.lower(): process(sub_parser, getattr(sub_parser, 'widgets', {}))
+                   for name, sub_parser in get_subparser(actions).choices.iteritems()}
   else:
     layout_type = 'standard'
     layout_data = process(parser, widget_dict)