add Mexico City

[bmount]

Mexico City has a pretty strong form of administrative subdivision, maybe relevant to this project. Most day to day local government happens at the delegación (borough) level. Personal identification of the residents as belonging to a particular borough is probably a bit less than it would be in New York but has some interesting aspects: for example Cuajimalpa or Coyoacán are far enough from the center and have enough of their own centers of gravity that they have pretty distinct identities from the rest of the city. I don't know enough about London or Tokyo to judge how analogous their subdivisions are but politically the delegación is very much like an NYC borough (if not stronger), and socially a bit weaker.

[utisz]

I'd definitely be in favor of this addition. The only data so far I could find on Mexico City so far is OSM data, which I'm a bit reluctant to embrace. OSM's boundaries of Tokyo wards were quite incomplete, and in some cases inaccurate.

[bmount]

I checked a few manually at geojsonlint.com and found reassuring details, but I don't have a great reference layer. I think the data may be ultimately derived from INEGI which is shockingly good most of the time, but doesn't have an open data policy per se. (There is an explicit global license on the UNAM site for non-profit and academic work and I think public government data may be reusable in other contexts too.) I could try getting more details from the professor who posted the material (email at top of link in readme) or if you'd like to reach out to him, he might find it more natural to work with a fellow @ dot edu.

[bmount]

If that was ambiguous, I mean the data in this pull request is from the UNAM source, and may be derived from a pre most recent census INEGI source. I think @lxbarth is working on getting the most recent census into OSM, which would be really reliable and great, but I didn't look at OSM for these boundaries.