Added MTL parsing support

Added MTL parsing support

• Modularized all of the source files into ES6 modules.
• • The Mesh, MaterialLibrary, and Material classes are now
    actual ES6 classes.
• Added tests for each of the classes
• • Found a bug in the Mesh class. Vertex normals would not appear
    if the face declaration used the shorthand variant; e.g. f 1/1
• Provided Initial MTL file parsing support.
• • Still requires Documentation. For now, have a look at the tests in the
    test directory for examples of use.
• • Use the new downloadModels() function in order to download the OBJ meshes
    complete with their MTL files attached. If the MTL files reference images,
    by default, those images will be downloaded and attached.
• The downloading functions now use the new fetch() API which utilizes
  promises.