Layer and ObjectGroup are parsing "visible" string, but not object.
TMX docs indicate expected to be "0" or "1"
https://doc.mapeditor.org/en/stable/reference/tmx-map-format/#object

fixes #47

Fixes #41

Not sure if the method is optimal, but here is some tile-flippin' goodness.

Also add test for parsed image layers.
The test can be also used as an example off how one can get data from
the parsed map.

this just adds a field to the Tile so that data may be parsed and accessed

Still need to parse properties.

The attribute is called "type" but we were previously looking for
"obj_type".