Restructured graph_objs_meta.INFO dict. Removed methods from graph_objs.

Restructure:
------------

This is the current structure in INFO:

```
    bar=dict(
        type=dict(required=True, type='plot_info',
                  val_types="default: type='bar'",
                  description="Plotly identifier for trace type, this is set "
                              "automatcally with a call to Bar(...)."),
    ),
```

Removed methods:
----------------

The following were removed from both PlotlyList and PlotlyDict:

* `get_json()`, no longer needed, automatically handled.

* `clean()`, used to remove None entries, no longer needed.

Other changes:
--------------

* `Trace` --> `PlotlyTrace`

* Changed `MetaDict` to reflect new graph_objs_meta structure

* Changed the way we check for `'valid'` keys to reflect new structure

* Changed the way we check for `'safe'` keys to reflect new structure

* `TYPE_TO_CLASS` --> `STRING_TO_CLASS`