spatial overlays

This branch implements polygon spatial overlays as discussed at the scipy 2014 sprints.

It leverages the work of the new spatial indexing to speed things up ... but it is still quite slow. The tests take ~40 seconds on my laptop and I'm sure we can do better. Profiling didn't reveal any low hanging fruit as most of the time was spent in shapely doing the union, polygonize and intersects.

There is an [example ipython notebook](http://nbviewer.ipython.org/github/geopandas/geopandas/blob/3384c82f80f8a099193aa0a74d9ff1c8d98e775d/examples/overlays.ipynb) to show the usage but no sphinx docs yet.

Note that this fixes #112 and lays the ground work for #135 

It probably could be considered a bug in `pandas.concat` that concatenating two `GeoSeries` gives a `pandas.Series` result.  It actually works for two `GeoDataFrame`s.

It would take a bit of digging around in pandas to come up with a fix.  Meanwhile, we could workaround with a `geopandas.concat` perhaps.