Updated API Doc

This reflects the changes made in the Google Doc.
New version of engine.json based on new schema

This isn't ready for primetime, but it is a start.
The new_engine.json is a concrete example of the
Construct --> Scheme --> Content approach I worked
out in the API Google Doc. It is a messy cut/paste
job that needs cleanup and testing but it's a start.
Updated engine.py and tests

Everything now works with the new engine.json.
This is a proof of concept commit. If I stick
with this structure for engine files, I will
need to make a number of improvements to
engine.py. More precisely, engine.py will
have to be smart enough to know how to make
a character without knowing which Schemes
to expect in the engine.json file.
Merge branch 'structure' of github.com:AgathaTheWitch/Otrera into structure