Add an armature reader

The armature reader reads in *.vtk or *.arm polydata files and outputs a
vtkArmatureWidget.
Also updating the BVH reader filename check.
Improve vtkArmatureWidget and vtkBoneWidget API

Add multiple transform methods (Scale, Rotate, Translate).
Return the value of the previous state when changing the widget state.
Add root accessors methods in armature.
Refacort FindBoneChildren to possibly search for all the children
recursively.
Massive WIP

TODO:
Factorize vtkArmatureReaderTest into a helper in armature.
Increase performance by using a map instead of a vector in armature.