added: exporter takes armature parent in account of animation handling.

only one armature source will can be taken at the moment, and will taken in the following "priority" order:
1. armature parent will be taken if exist.
2. first available armature modifier will be taken if exist.

open question: what is best practice, in case of existence of multiple armature sources?
(armature parent and one or more armature modifier)
can that case happen (very often) in real world mesh modeling?