Use type() to create new classes and a factory to instantiate.

Some organizational changes. Previously, if you wanted to add a new trace say, TreeChart, you'd need to add it to graph_references, add a class definition in graph_objs.py, add multiple mappings in graph_objs.py and add it to a couple __all__ lists... which is a lot.

The reason we need to dynamically create classes is because we never know when graph_reference has changed! Therefore, we just do it at import time... Therefore, we were using metaclasses, but this is a little overkill, we can just pass a compiled `__doc__` to `type()` to make a class for us. The doc won't be changing after this, so it's not a problem afterwards.
ditto with KEY_TO_NAME... except, we don't actually have a one-to-one mapping... it's n -> m where n > m. yikes! 

I might suggest that this is an unnecessary complication and we should just add 'textfont', 'titlefont' and 'tickfont' as separate objects in graph_reference. I think that's probably most consistent anyways...