Armature bugfixes, animation refactor for constraint, NLA support

I've applied my earlier changes to your latest.  This adds the following features:

 - Armature export bug fixes

 - Animation refactor, now reaches into constraints to determine if there are animation tracks that need to be baked

 - Support for exporting NLA tracks, each as a separate OSG animation track.  This enables the user to use the Blender NLA editor to organize actions into groups of animations that are meaningful for the target application.  (this isn't exactly what the NLA editor is intended for, but it works reasonably well).

 - Can now specify the filename extension to convert to when calling out to osgconv.