df_to_geojson() function should JSON-safe stringify all Pandas datatypes

[ryanbaumann]

Problem

Passing a datetime Pandas datatype column to df_to_geojson() as a property output throws a JSON serialization error.

See https://github.com/mapbox/mapboxgl-jupyter/blob/master/mapboxgl/utils.py#L28

Solution

Add a data type check and JSON-safe stringification method for all Pandas datatypes datetime64, timedelta[ns], and boolean .