Fix for https://github.com/KhronosGroup/OpenCOLLADA/issues/207

Hello.
I believe this patch solves the issue mentioned in my post from 2 days ago.
I tested with the Blender Collada importer (development version) that the patch works as intended.
I refer to the Blender bug report (which includes an example file):

https://projects.blender.org/tracker/?func=detail&atid=498&aid=36325&group_id=9

Further notes:
The specification 1.4.1 does not tell that <skeleton> entries are mandatory. 
Furtherore the FBX Collada exporter doesn't generate <skeleton> entries for Library Controllers.
And finally Maya-2013 accepts the collada file and shows the model correctly.

Please can you commit, rework, or reject this patch in the near future ?
As soon as the patch is applied we can update the Blender Collada Importer as well.

Thanks for helping
Gaia