Added subplot instantiation for Figure objects in graph_objs. NB update.

You can now instantiate Figure objects with a `subplots` keyword:

```
Figure(subplots=[3,4])
```

This assumes that `subplots[0] == rows` and `subplots[1] == columns`.

For more control, you can alternately include a `subplots` dictionary:

```
>>> subplots = dict(rows=2,
>>>                 columns=3,
>>>                 horizontal_space=0.05,
>>>                 vertical_space=0.1,
>>>                 get_grid=True)
>>> fig = Figure(subplots=subplots)
This is your grid!
[4] [5] [6]
[1] [2] [3]
>>>
```

The `get_grid=True` response is a little dopey, but it's pretty helpful
if you just want to know which plot you need to reference.

Also, TestDrive.ipynb is updated to reflect changes.