ENH: Spatial join

Implement spatial joins; closes #115

Examples found in the [notebook](http://nbviewer.ipython.org/github/geopandas/geopandas/blob/sjoin/examples/spatial_joins.ipynb)
My bad - I thought I had `--assume-unchanged` that file but it slipped through. I think it's a good change but doesn't necessarily belong in this pull request.