Fix a comment mentioning the old changeToScene() function

We have replaced this with Scene:changeTo() so the source-code
comments need to reflect that.
Provide a utility function for loading scripts

This will be useful for games using LNVL so that they do not have to
write their own logic for loading scripts and assigning the scene.
And it will give us a useful place to insert debugging code.
Give an example of using LNVL.loadScript() in the main script
Remove outdated information from the file-header in the main example
Show all scene opcodes when in debugging mode

Currently we only show the opcodes for Scene objects which we assign
to the LNVL.currentScene global.  But that does not work.
Furthermore, we may later want to have a metatable on ‘LNVL’ for other
reasons and using one only for debugging would get in the way.  It
will be more convenient if we show the opcodes for scenes inside of
the Scene:new() constructor when using debugging mode.  That way we
are sure to see the opcodes for every scene, and it keeps
scene-related debugging code closer to the rest of the scene code.
Update the instruction documentation about showing scene opcodes
Document why the example images are missing from the repository
Merge branch 'loading-scripts'

* loading-scripts:
  Update the instruction documentation about showing scene opcodes
  Show all scene opcodes when in debugging mode
  Remove outdated information from the file-header in the main example
  Give an example of using LNVL.loadScript() in the main script
  Provide a utility function for loading scripts