WIP: simplifies and tests

[stsievert]

Work to be done before merge:

• implement new dashboards (basics implemented, screenshot below)
• rewrite dashboards: "encode [column] as [encoding]" (i.e., "encode species as color")
• rework the data flow (and untangle the spaghetti code)
• allow other specific parameters in advanced (i.e., in scale allow kwargs bin, zero, etc).
• allow global parameters to be set (i.e., for heatmaps allow applyColorToBackground)
• implement command to get Altair command out to generate the shown plot
• test

[stsievert]

All the tasks are complete except one (detail below). This PR is nearly ready for merge and I feel the current implementation is fairly stable.

@jakevdp can I hear your thoughts on naming? I only expose interact_with(df, ndims=3, show=True) but it also possible to access the class behind all the widget, Interact which has minimal public elements.

I implement 13 tests, some doctests and some testing interfacing with Altair (can't test UI yet, #5 is an example bug). I implemented the tests in a notebook because of an issue (jupyter-widgets/ipywidgets#13?) that means I can't run tests from the command line and only in Jupyter notebook. While not ideal, I've interfaced these notebook tests with pytest and Travis CI.

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Screenshots

General interface (an older GIF can be seen on the README for this branch):

When the encoding options are expanded:

When the mark options are expanded:

I have filed jupyter-widgets/ipywidgets#861 about the checkbox width.

[jakevdp]

Quick question – would there be any way to make it so the ndims is not pre-specified? I imagine it being rather annoying if you're exploring a dataset, find you need one more dimension, and then have to re-run the cell and re-enter everything you've done.