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.
definitely a philosophical argument. i'll leave it *as-is* for now, and we might open this back up down the road.

My major argument is that if all of these objects point to 'Font' as an example, 'textfont' *and* 'titlefont' have to have the same parameters. This is OK, if we're *sure* of it. We're just giving up a little bit of flexibility on it moving forward.