Adding creation of geo directory before trying to write to the directory

Hello there, my colleagues and I have learned a great deal from your tutorials located on http://bost.ocks.org/mike/ and we thank you for writing them.  I have found a bug in the Makefile for the world-atlas repo and would like to bring it to your attention.  Regarding details on the bug I found, while running make for the first time I get the following error.

	rm -f geo/ne_10m_us_states.json && ogr2ogr -f 'GeoJSON' -where "iso_a2 = ('US')" geo/ne_10m_us_states.json shp/ne_10m_admin_1_states_provinces.shp
	ERROR 4: Failed to create GeoJSON datasource: geo/ne_10m_us_states.json.
	GeoJSON driver failed to create geo/ne_10m_us_states.json
	make: *** [geo/ne_10m_us_states.json] Error 1

It appears that the directory named 'geo' needs to be created just before writing the output of the ogr2ogr command to a file inside of it.  This branch attempts to do just that.  I'm looking forward to more of your tutorials and checking out your new projects.  Thank you.