Fix problem with rotated meshes.
Fix problem with rotated meshes.

The problem was that assimp rotates collada files such that the up-axis (specified in the collada file) aligns with assimp's y-axis.

dartsim/dart,dartsim/dart,dartsim/dart,axeisghost/DART6motionBlur,axeisghost/DART6motionBlur,axeisghost/DART6motionBlur,dartsim/dart,dartsim/dart