Change Stream meta

- Stream (graph object, linked in trace objects) 'type' set to 'object'
- 'titlefont' 'value' set to 'object'
- add 'object_list' item to `val_types` shortcut dictionary
- set 'annotations' (in Layout) and 'data' (in Figure) 'value' to 'object_list'
Merge pull request #16 from plotly/trace-stream

Change Stream meta