Convert the MML file from JSON to YAML

This one-time conversion was done with the json2yaml node module,
and then substantially reformatted by hand, using mappings and alias
to avoid repeating projection and extents information for each layer,
and to get consistent ordering of the properties of each layer. The
latter does not matter semantically, but makes editing with a text
editor cleaner.

By using the scripts/yaml2mml.py script, the YAML can be converted
back into JSON, which is still what TileMill uses.

A JSON-aware diff of the original project.mml and the new generated
one shows that the only differences are the new "_parts" property,
used for aliases and ignored by TileMill, and layer extents.

The script can be invoked with

    scripts/yaml2mml.py < project.yaml > project.mml

The project.mml file remains checked in to git, as TileMill requires
it and this allows someone to clone the repo, and immediately have
it work. Merge conflicts in project.mml are trivial to solve, as
the entire file is just regenerated by running the script again.

This does mean that the .MML file should not be edited directly,
but this is fine, as editing the JSON by hand is extremely difficult
anyways! The HOT OSM style has a similar workflow, making use of cartocc.