Allow SelectBoxes, Radio Buttons, and MultiSelect to use DataFrames

[kmcgrady]

Issue: #78

Description:
When a DataFrame is sent as an argument, the component will select options from the first column.

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[tconkling]

Looks good!

Nit: ensure_iterable feels like the wrong name for this new function, since it doesn't actually ensure that its input is iterable unless that input is a dataframe.

(If I passed some random non-iterable object to ensure_iterable, I'd assume that it'd either return a new object that was somehow iterable, or throw an exception.)

Something like make_iterable_if_dataframe would be clearer (if verbose and clunky sounding)

[kmcgrady]

Thanks @tconkling ! My only thought was if we decide to support more list-like types, we would just add to the ensure_iterable function. I could just throw an exception if iter(obj) throws an exception. It should then work in all the other cases. Thoughts?

[tconkling]

Throwing an exception seems reasonable to me!