Parse marker

Parse markers on BodyNode. BodyNode can contain multiple markers.

Here is an example `skel` file for markers.
```xml
<?xml version="1.0" ?>
<skel version="1.0">
        <skeleton name="box skeleton">
            <body name="box">
                <marker name="box_marker1">
                    <offset>1 2 3</offset>
                </marker>                                
                <marker name="box_marker2">
                    <offset>-1 -1 -1</offset>
                </marker>                                
            </body>
        </skeleton>	
</skel>
```