Geopandas explorer

basically we monkey patch geopandas to have an explore method that calls dframe_explorer with all the right configuration.

This is nice because geopandas
* can convert to 4326
* has the center of the shapefile already
* can read shapefiles rather than geojson into a leaflet map
* can convert to geojson on the fly so that no file on the filesystem is required

This also calls the webbrowser automatically when everything is ready