Patch release FBX2glTF v0.9.1

The first patch release of FBX2glTF brings a small list of urgent fixes and pleasant improvements:

• Texture coordinates are now vertically flipped by default, as per #21. In the common case, this is a required translation between the FBX world and spec-compliant glTF.
• Texture image files are now resolved on the local filesystem with more discipline, and clearer warnings when they can't be found. When generating .gltf rather than .glb, we additionally copy the texture files into the output folder, and reference them appropriately in the model file. See #16 and #23 for details.
• The Node API has been much improved, as per #10.
• The main README has been given some love, and should now be a more dependable source of information.