Add example for working with OSM path data.

What I'd like to do is extracting "features" of street systems (such as
typical length of street segments, number of turns, angles), analyze the
results and use them to get a better idea how to procedurally generate
somewhat realistic looking street systems. (And to write more code in
clojure, of course.)
Add Makefile to get path data.

Also, `dev-prepare` should be able to use any file you want as an
argument (because I'm lazy).
Convert tag keys to keywords if possible.