Make graph_objs.py compatible with new graph reference!

- created from @theengineear 's factories branch
- adapt graph reference INFO to new graph reference structure
- add doc content (e.g. links to plot.ly examples and parent keys for a given obj)

i think it's usually better to do file stuff like this through os.path:

```python
graph_reference_dir = 'graph_reference'
json_files = [
    'graph_objs_meta.json',
    'OBJ_MAP.json',
    'NAME_TO_KEY.json',
    'KEY_TO_NAME.json'
]
for filename in json_files:
    relative_path = os.path.join(graph_reference_dir, filename)
    s = resource_string('plotly', relative_path).decode('utf-8')
    # etc...
```

It doesn't bother me to no capitalize CONSTANTS, like 'JSON_FILES' or 'GRAPH_REFERENCE_DIR', but you could certainly do that too. I supposed we do have a convention sort of set here for doing that and we should be consistent.