ENH: add Chart.serve() command to serve HTML

[jakevdp]

This PR adds a functionality to create altair plots from outside the Jupyter notebook; e.g.

from altair import Chart, load_dataset

cars = load_dataset('cars')

chart = Chart(cars).mark_point().encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color='Origin',
)

chart.serve()

The final line will create a local web server, and by default launch a web browser window showing this visualization.

[jakevdp]

I'm going to merge this; we can iterate later if needed.