Implement union and difference methods.
Implement union and difference methods.

kwinkunks/geopandas,jwass/geopandas,micahcochran/geopandas,geopandas/geopandas,snario/geopandas,geopandas/geopandas,jwass/geopandas,ozak/geopandas,perrygeo/geopandas,jorisvandenbossche/geopandas,urschrei/geopandas,fonnesbeck/geopandas,maxalbert/geopandas,micahcochran/geopandas,ozak/geopandas,IamJeffG/geopandas,koldunovn/geopandas,jorisvandenbossche/geopandas,jdmcbr/geopandas,geopandas/geopandas,jorisvandenbossche/geopandas,scw/geopandas,jdmcbr/geopandas