an AssimpModel util to control skinned mesh loaded with ofxAssimpModelLoader

Pull request for :
* an AssimpModel util to control skinned meshes loaded with ofxAssimpModel.
* an example that shows how to use the util and provides an example Blender file to show the armature setup.

Bones orientations are applied using euler angles and some not-so-pretty translation of coordinate systems (that I can't really explain), but seemed to work with all combinations I've been able to try so far.
Demo : http://www.flickr.com/photos/kikko_fr/9211100150/