Custom Types Support

[xivol]

It seems that support for custom types in Tiled Project files is broken.
The code is all there for it to work but it doesn't pass the right arguments around.

546            self.parse_json(json.load(open(custom_property_filename))["propertyTypes"])

559        TiledElement._set_properties(self, node, self.custom_types)

1262            p.update(parse_properties(child, self.parent.custom_types))

I fixed it in couple of lines, should we do a PR or can the author just fix it? Idk how community works here.