Remove boilerplate from API

[jakevdp]

This is a tiny change in the top-level API that makes for less boilerplate. Using the scatterplot example, the current API looks like this:

Layer(data).encode(
    X('Horsepower'),
    Y('Miles_per_Gallon'),
    Color('Origin'),
    Size('Weight_in_lbs')
).point()

You can also use keywords, e.g.

Layer(data).encode(
    x=X('Horsepower'),
    y=Y('Miles_per_Gallon'),
    color=Color('Origin'),
    size=Size('Weight_in_lbs')
).point()

With this PR, there is a (to my mind) more intuitive way to do this:

Layer(data).encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color='Origin',
    size='Weight_in_lbs'
).point()

The string arguments are converted automatically to the appropriate object. Note that this works only in the encode() function, not deeper within the object hierarchy.

What do you think?

[ellisonbg]

Nice, do the shorthands work in the strings?

[jakevdp]

Yes – x='foo' is exactly equivalent to x=X('foo')

[jakevdp]

We might also think about allowing this kind of shortcut deeper down, though that would be a bit trickier.

[ellisonbg]

Looks good, merging.