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
Think we should leave this as an optional dependency so it doesn't gum people up who don't plan to use it?