Add contrib module to explore interface with geopandas

[sjsrey]

https://github.com/kjordahl/geopandas

[ljwolf]

It seems that most of this works together rather easily, given more arbitrary W procedures. With the proposed changes in wconstructor for geointerfaces, a pysal.weights object could be constructed from any GeoSeries.

I've been playing around with what's possible given this W from GeoPandas objects. esda gains instant support, as calling geodataframe[column].values yields a (n,) numpy vector, so every pysal.Moran* works using Series' .values methods. Compatibility holds using this in spatial_dynamics, too.

We still run into issue #461 for spreg, as gdf[col].values has to be reshaped for y but not X. The fact that pandas provides column vectors as (n,) might indicate a good external standard to peg on for this issue.

That said, once the new wconstructor gets tested, doing this might be better off as a (highly visible) set of example notebooks.

[ljwolf]

Formal start for contrib module in PR #618.