Feature Request: honor size argument from mlab.figure in notebooks

Presently, the size of the figures in a notebook environment is determined by the initial call `mlab.init_notebook(width=800, height=600)`.

It would be convenient to set the figure size for any given figure like so:
```
fig = mlab.figure(size=(100, 100))
t = np.linspace(0, 4 * np.pi, 20)
x = np.sin(2 * t)
y = np.cos(t)
z = np.cos(2 * t)
p = mlab.points3d(x, y, z, figure=fig)
fig
```