Added helpful error for PlotlyList instantiation.

Before this raised a confusing *validation* error:
```python
Data(Scatter(x=[1,2,3], y=[2,1,2]))
```

The reason being that if you unpack a dictionary into args (`*args`),
you get a list of *keys*, which is never going to make a valid Plotly
graph object. Therefore, we just catch it and raise a more helpful
*instantiation* error instead.
Updated tests which now raise errors (this is a good thing).
Merge pull request #57 from plotly/plotly-list-instantiation

Plotly list instantiation